by John Richardson

I’ve been on something of a roll lately with amplifiers from Pass Labs. Readers might recall that I reviewed a monster of an amp, the Pass X250.8 a couple of years ago, and then more recently, Pass Labs’ lower powered integrated offering, the INT-60.  Both were fine examples of top-drawer modern solid state amplification, and while I found the INT-60 to be a bit pickier about speaker pairing, the brutish X250.8 could drive just about anything and always look great doing it, with no loss of composure whatsoever.

Well, guess what? I’ve got another one for you: a fine pair of Pass Labs XA60.8 mono block amps. These mono beasts are a different flavor of Pass than I’m used to, as they actually put out their full rated power in class A, as opposed to the others I’ve spent time with, which operate in class A for some fraction of their rated output and then shift into powerful (but less refined) class AB mode.  I don’t want you to get the idea that I’m not familiar with good class A designs from Pass Labs head honcho Nelson Pass, as I’ve spent lots of time with such implementations from Mr. Pass’ other company:  First Watt.  Amps from that establishment, all class A designs, that I’ve enjoyed include the SIT-1, J2, and F7, which are all excellent in their own right, but offering much less power than their typical Pass Labs cousins.  If you decide to go the First Watt route, speaker matching becomes of paramount importance.

My experience with Pass Labs amplifiers is that they most definitely possess a “family” sound.  Each one is sort of like a chef making variations to an otherwise perfect recipe, just to meet the taste requirements of certain special clientele.  I’ve found it hard to fault the general sound of these amps (though I’m sure they have their detractors somewhere out there…), not to mention their overall performance with a variety of different speakers, their reliability, and sterling build quality.  My dad, who has been an excellent cook his whole life, says that if you want to take a very good lettuce salad to the next level, all you need to do is add a dash of sugar.  Maybe that’s what Pass Labs has done in the case of the XA60.8, as I can say it’s now my favorite Pass Labs product I’ve heard to date.  Yeah, that must be it: the full class A implementation must be the equivalent of a dash of hi-fi natural sweetener.

So then, you might ask, what is the Pass Labs “family” sound?  Different people hear things differently, but for me, it goes something like this:  it’s somewhat warm, well-balanced top to bottom, dimensional, and forgiving, all while demonstrating prodigious bass.  In fact, it may well be the bass thing people notice first.  I know that when I demoed the XA60.8 amps for an audiophile friend, he kept coming back to the bass and how good it was, as in deep, textured, and defined.  What I really like about the Pass Labs amplification “sound” is its long-term listenability.  I’ve found that with the XA60.8 monos in my present system configuration (which I will share with you shortly), I can put a new album on repeat, sit in my chair, close my eyes, and come back to reality three hours later without realizing I’ve heard the album possibly three times through.  With each repeat, there’s something new to analyze and chew on, without the music getting stale or my ears getting tired.  In my own experience, if a system can treat me that well, then it’s due some pretty high praise from this reviewer and audio enthusiast.  Distilling it all down then, I think it’s ultimately the long-term, non-fatiguing sonic friendliness that gets the Pass Labs products such high marks in my book.  I’m talking about smooth, non-peaky treble; nice, slightly lush, warm midrange; and that oh so lovely prodigious but well-tuned bass response.

While Pass Labs amps most definitely have their own sonic signatures, they are also especially good at conveying the sonic characteristics (either good or bad) of any upstream components you choose to use along with them, as we soon shall see.

So, let’s now look a little deeper.

Getting to Know the Pass Labs XA60.8

Quoting Pass Labs’ marketing department:  “…. The XA power amplifiers have remained at the cutting edge of audio through continuous innovation and refinement.  At Pass, we build amplifiers with excessive output stages, huge heat sinks, and massive power supplies.  We use the highest quality components in simple linear topologies, assembled and tested with great care in Auburn, California.  They measure well and sound great.  They are heavy, reliable, and expensive.”  That about sums it up — review over!  Just kidding…

As the name would suggest, the XA60.8 puts out a solid 60 watts per channel into an eight ohm speaker load, doubling its output with decreasing impedance to 120 watts into four ohms, which is pretty much what one would expect from a Pass Labs amplifier.  Build quality and overall aesthetic are also top-notch and in line with everything else I’ve seen from the company.  These are components that inspire confidence and pride of ownership, exactly as they should for the asking price of $13,500 per stereo pair for the XA60.8.

If you care to look at the really technical details of the XA60.8 such as slew rate and damping factor, then hop on over to the Pass Labs website and have a look.  Me?  I’m far more interested in what it’s like to get these guys set up and how long it takes for them to run in and start sounding good.

First off, when amps like these show up at my front door, I find them to be sort of head-scratchers due to their sheer weight. They are heavy, and I mean it.  Nelson Pass once joked that it takes a band of ruffians to get these amps unboxed and properly set up.  Specifically, each XA60.8 weighs somewhere right around 90 lbs, and unfortunately for my back, there are two of them.  I managed to get them un-boxed myself, but then it took my son and me some considerable huffing and puffing to get them up two flights of stairs and finally situated in my attic listening environment.  These are definitely the kinds of components you want to place only once if you can help it.  Nelson says he can still move a single XA60.8 by himself, but just barely.  My hat’s off to him.

Once the amps are set up, the next big hurdle is the breaking-in stage.  I’ve found that it can take considerable time to get Pass Labs amps sounding their best.  Oddly, the XA60.8s sounded exceptionally good right out of the boxes.  Unfortunately, the good news didn’t last.  By the next day, they were cold and ragged sounding by comparison, but with continued use, the amps improved slowly and steadily.  I left them running continuously for four or five days, making sure not to look at the rate of spin on my electric meter.  Listenability was considerably better after that and continued to get better well past 100 hours into the experiment.

Another drawback for amps running in pure class A such as these is that they get hot; and again, the issue is compounded since there are now two of them.  I received the amps in July, which is typically the hottest month here in Pennsylvania.  Even though we have central air conditioning, the attic stays fairly warm during those summer months; therefore, I had to use excessive patience with regard to running the Pass amps. Finally, by early August, we got a break in the heat and were finally off and running.

Once properly run in, I kept the amps in standby mode when I wasn’t using them.  My usual routine goes something like this:  I turn them on when I get home from work and then go downstairs to fix a martini and prepare dinner.  Once the dishes are done, I sneak back upstairs for some listening time, with the amps having had a good two hours of warm up.  They then get turned back to standby come bedtime.

For the sake of review, I used the amplifiers in two configurations.  The first involved driving them directly from the balanced outputs of my Antelope Zodiac DAC equipped with a custom linear power supply, which allowed me to take advantage of the theoretically lowered noise floor offered up by using balanced connections.  For a bit different presentation, I opted to run single-ended into the Pass amps from my Linear Tube Audio MicroZOTL2.0 serving as a preamp.  Feeding the inputs of the MicroZOTL2.0 were either my Antelope Zodiac DAC or the excellent BorderPatrol DAC (SE version; via S/PDIF input).  This DAC, reviewed here, is both non-oversampling and non-upsampling, and offers an organic, yet detailed sound signature, albeit at Redbook resolution only.  Speakers used during the evaluation period included ATC SCM19 (version 2) monitors, as well as efficient single-driver models from Zu Audio (Omen Mk.2, Dirty Weekend) and Omega Speaker Systems (Super Alnico Monitors).

Unlike the Pass Labs INT-60 integrated amp, these 60 watt per channel mono blocks driven directly by the Antelope DAC had no issues whatsoever providing plenty of slam, speed, and dynamics to my ATC speakers.  Pretty much a “grab ‘n’ go” scenario here:  the amps essentially grab the speakers by the balls, um, woofers, and get the show going.  Tonality is pretty much in line with what I remember the X250.8 provided, namely smooth, controlled, slightly warm and rich sound all around.  If anything, I’d have to say that the XA60.8s may be a bit sweeter in tone, primarily through the mids and treble.  I’m fortunate to have the same X250.8 amp I reviewed in 2015 back in the house, so I’ll get into more specific points of comparison a bit later.  As good as this setup is, however, I’ve found it to sound just a wee bit mechanical, especially in the upper mids and treble.  Nothing terribly unpleasant, mind you, but noticeable over the long haul.  Fixable?  Heck yeah!  Time to move on to evaluation setup number two, the one with the tubed preamp.

If the XA60.8 monos sound good on their own, they sound fabulous with the LTA MicroZOTL2.0 tubed pre and either the Antelope Zodiac or Border Patrol DAC on the front end!  This setup is the one I referred to earlier that I could listen to and get lost in for hours on end, with no fatigue or frequency dependent nastiness at all.   OK, so I did have to give up the advantages afforded by balanced connections, but the tradeoff for warmer, smoother, and even more dimensional sound was, at least to me, more than worth it.  My point here is that the Pass Labs amps do a phenomenal job of passing down to the speakers the sonic characteristics of whatever gear comes before them in the audio chain.  Of course, opinions will vary, but the aforementioned setup brings a new level of long-term audio enjoyment into my world.  I fear my wife won’t be happy in her new-found loneliness, or I’ll need to become a more sensitive spouse and do more late-night sessions after she goes to sleep, remembering to keep the volume down to reasonable levels.

Let me explain myself a bit more explicitly here.  Audiophiles value the various aspects of musical reproduction (and I don’t mean copulating to nice tunes here) to differing degrees.  As for myself, I tend to place accurate tonal density along with pace, rhythm, and timing (PRaT) at the top of the list.  I greatly appreciate the cool effects of three-dimensional stereo imaging and bone-jarring dynamics as well, but if the previously mentioned two attributes are lacking, then all bets are off for me.  Of course, no stereo setup is going to perfectly mimic a live performance, so my benchmarks for tonality and PRaT must come from some other source.  I’ve been dipping my toes into the world of single-ended triode vacuum tube amplifiers lately and have become enamored by what these little wonders can do within their limitations.  Two things a well-implemented SET amp can do, wonder of wonders, are tonality and PRaT.  An amp I have here that really gets me going is one of Dennis Had’s (of Cary Audio fame) hand-built high output Inspire SET models.  The one I have is a custom job that delivers somewhere around 10-15 watts of glorious tube sound using 6550 output tubes.  While maybe not an absolute benchmark for tonality and immediacy compared to the real thing, I’ve found this amp to establish a nice baseline for comparison against other amplifiers with respect to tone and PRaT.

Now that I’ve got the Pass mono blocks tuned exactly to my liking as fed by the LTA preamp, we’re ready to talk some specifics.  As I’ve mentioned in a few other recent reviews, streaming audio via Tidal/MQA has been a game changer in how I access and listen to music.  Most of my old favorites are right there at my fingertips, as well as a hugely accessible vault of music I’ve never before heard.  I’m like a kid in a candy shop; correction: a really huge candy shop.  For those who need hi-rez (I don’t, though it is nice to have from time to time), the selection of MQA albums seems to have mushroomed just in the last month or so, and I can’t imagine the flow slowing anytime soon.  So far, I haven’t felt the need to go after an MQA capable DAC, as Tidal’s playback software gets you at least partway there in terms of decoding the MQA files.

So then, thanks to Tidal, let’s get down to business.  Now, just where should we start?

Ah, let’s begin with the interesting Miles Davis recording of Kind of Blue Legacy Edition.  Yep, this is the classic audiophile jazz chestnut along with some interesting extras: some alternate takes, false starts, and even some extra tunes that weren’t on the original album (these are included in the Volume 2 section).  I’ve always enjoyed Kind of Blue, but the Pass Labs XA60.8 amps draw me into the album in ways I’ve not experienced before.  It’s the tonal purity and spaciousness of the recording that really grab me, from Miles’ sharp, pungent horn to the smoothness of the sax and finesse of the drum kit.  I can hear exactly what’s going on in the recording, without it ever becoming tonally etched or overly dry.  Conversely, there’s scads of liquidity and tonal color here.  I wouldn’t exactly refer to what I’m hearing as a warm, wet wall of sound, but we’re definitely closer to that end of the spectrum, especially with the tubes up front.  The experience is riveting enough to keep me coming back to this favorite time and time again.

A huge benefit of having a Tidal subscription is being able to explore music I would otherwise never get to hear.  Yeah, there’s some odd but good stuff out there.  One interesting selection I stumbled upon by sheer luck is Belgian jazz artist Rony Verbiest’s album, Rony Verbiest Plays Dave Brubeck.  As the title suggests, this is a tribute album to the great jazz pianist and composer, but with a funky twist.  After a bit of fact-finding, I discovered that Mr. Verbiest is a modern jazz musician who specializes in two instruments:  the baritone sax and the accordion.  Yes, you read that correctly.  I think the last time I heard someone really go to town on the accordion was while watching the Lawrence Welk show with my grandmother years ago.  Cheesy, I know, but somehow this album really works.  And it’s wonderfully recorded.  To get a feel for what’s in store, just check out the first cut, “Here Comes McBride,” and you’ll be rewarded with some spectacularly recorded jazz fireworks.  With two accordions to boot!  If you have Tidal, check it out and tell me what you think, as you won’t find a physical copy of this anywhere for a reasonable price (trust me, I looked).  At any rate, my system with the XA60.8s in place renders the performance with a wide, deep soundstage that just pops when I close my eyes and get lost in it all.  Lots of space and air around the instruments, yes, as well as that lovely tonal structure I keep coming back around to, with the lower notes and undertones of the baritone sax plumbing right down into the depths as Pass Labs amps are wont to do.  We’re talking bass with real finesse to go along with extension.  PRaT must be pretty good too, as my head bobs and my feet tap in response to Verbiest’s cool rendition of the famous tune “Take Five.”  The interplay between the saxophone and the dueling accordions is actually quite mesmerizing on this cut when properly played back, as it most definitely is here.

Getting away from jazz for a bit, another album I’ve been enjoying is Response, with solo percussionist Odd Borge Sagland (hope he wasn’t tormented too much as a kid…) performing later 20th Century percussion works by Arne Nordheim, Toshimitsu Tanaka, and others (BIS, streamed via Tidal).  On display here is a whole carnival of percussion instruments, including bongos, timpani, marimba, vibraphone, and others.  Well recorded percussion is useful to me as a reviewer, as it tells me a lot about how an audio system handles electronic reproduction of attack, decay, dynamics, and timbre.  In none of these areas did the XA60.8 driving my ATC speakers disappoint in any way, and I was particularly impressed with the depth of soundstage of the solo marimba in Tanaka’s “Two Movements for Marimba.”  The whole recording takes on an airy, almost ethereal aura that was beautifully portrayed using this system combination.

Regardless of what type of music I chose to hear, it was the long-term enjoyment and listenability that the Pass Labs XA60.8 amps when coupled to my ATC SCM19 speakers that kept me coming back again and again, or perhaps really never wanting to leave my listening room in the first place.

Pass vs. Pass:  The X250.8 vs. the XA60.8 Monoblocks

Nothing like a little sibling rivalry, right?

While the ATC speakers were still in the system, I wanted to find out how the Class A Pass XA60.8 mono amps would compare against the stereo Class AB Pass X250.8 amp.  The latter amplifier, when it makes the occasional visit to my home, has pretty much established itself as king of the hill when asked to drive my ATC SCM19 (version 2) speakers.  I’ve found these speakers to be a lot more accommodating to various types of amps than I would have ever thought, but most people who own them like to have lots of power (as in well over 100 watts per channel) on hand.  Indeed, the Pass Labs X250.8 makes an excellent case for that scenario.

My immediate impression was that the X250.8, like the XA60.8s, benefitted sonically when used in cahoots with the LTA MicroZOTL2.0 tubed preamp.  Again, increased warmth and dimensionality prevailed, giving the amp a bit kinder and gentler overall presentation.  Differences were subtle, but noticeable, especially after a few hours of listening to the X250.8.  Bass seemed to go even deeper (if that’s possible), and during crescendos, the “louds” seemed “louder,” suggesting an improvement in macrodynamics.  My ATC speakers really didn’t seem to mind the extra watts in the bank at all.  On the plus side of the ledger for the XA60.8, I’d have to say that I still had a slight preference for how those amps did tone and timbre.   These attributes just seemed a little bit more natural and unforced, leading to a little bit less long-term listening fatigue.  Both amplifiers performed remarkably well with the ATC speakers, and probably finished in a dead heat for this listener.  Which will work best for a particular enthusiast will depend on listening preference and speaker choice.

The XA60.8s and Efficient Speakers: Driving the Zu Audio Omen Dirty Weekends

I’ve been getting a great deal of mileage lately out of my recently acquired Omen Dirty Weekend speakers from Zu Audio.  I love how these things sound, and they represent a tremendous value; you just have to get ‘em when they’re hot, which is two or three times a year.  Is driving a $1000 pair of speakers with $13,500 worth of amplifier crazy?  Yeah, probably, but why not?  The Omens have responded beautifully to just about every amp I’ve tried them with, so I’d expect the Pass amps to really get them cooking, especially at their published 97 dB sensitivity.  Power?  Yep, we got that covered.

The XA60.8s wove some beautiful musical tapestry with the Zu speakers, especially through the midband.  The Omens are particularly expressive through this presence region, and this strength was brought straight to the forefront.  I noticed this particularly when listening to Miles Davis and his band of merry gentlemen doing their thing on Kind of Blue.  Both the trumpet and saxophone were palpable and lively enough to literally give me goosebumps.  Bass was nicely extended, but lacked the vise-like control that the ATC speakers offered.  A little flabby in the bottom possibly, but still mightily enjoyable, and something my ears became accustomed to quickly.  I sometimes forget what champs the SCM19s are in the bass control department.

Remember the Dennis Had Inspire SET amp I mentioned earlier?  Well, now that some efficient speakers were set up, it was time for the ultimate comparison.  How close could the XA60.8 amps really come to a real SET tube amp in terms of their conveyance of tonal accuracy, immediacy, and PRaT?  Let’s find out.

It should come as no surprise that the efficient Zu Omens really sang with the Inspire amp.  What I heard was more palpable texture; more colorful, fleshed out tone; and an expanded soundstage.  Not only did the soundstage seem pumped up, but the instrumental images within it grew proportionally in volume.  What I got was everything I expected to get with a SET amp from a top designer in that arena with the caveat that such an amp must be paired with an appropriate set of speakers.  I’ve driven my ATCs with the Dennis Had amp, and it works, but much of the magic is gone.

The Pass Labs amps, on the other hand, still offer some advantages when driving the Omens.  These include deeper, better controlled bass and an overall feeling of supreme control over the speakers, though some of the tonal magic offered by the SET amp is absent.

OK, Let’s Sum It All Up

Like all reviews, I hope that this one nails down one important point that bears repeating again and again:  no audio system or component is perfect, even within the limited realm of home audio reproduction.  It’s all about component matching to best meet each listener’s perception of perfection, whatever that may be.  It’s all about trade-offs, and which ones are most acceptable to you, the listener.

With this point in mind, here’s what I learned.

First, the big Pass Labs amps (here, both the XA60.8 and the X250.8), at least to me, sound best when fed with a vacuum tube preamplifier.  Such an arrangement takes the Pass Labs strengths and takes them to a new level of enjoyment.

Second, while the XA60.8 was a hit with all speakers I matched them with, the real magic showed its face with the amps driving my ATC SCM19 speakers.  This combination represented a near perfect case of “refinement meets refinement” which really knocked the ball out of the park for me.  This was the ultimate pairing I kept wanting to return to throughout my time with the amps in-house; it’s the one where I was most likely to get lost in the music and forget I was listening to a stereo setup in the first place.

Lastly, speaking of trade-offs, one sly trick the XA60.8 amps pulls off quite effectively is to get me a good way toward the SET sound I love, but without the limitations.  Not quite there, mind you, but close enough that I could revel in the improvements in sound offered in other areas such as dynamics and bass control without worrying about what I may have been missing in ultimate tonality.  Oh yes, and think of all the different speaker combinations one could try that would cause even a thoroughbred SET amp to shudder in abject fear.

Expensive? Yes!  But very much worth it if you’ve got the green and the system to keep up with what these XA60.8 amps can do.  Another triumph from Nelson Pass and the design crew at Pass Labs.

About the Author

John has been interested in music and audio since his early teen years, or stated another way, as long as he can remember.  He has been involved in the audio community in one way or another for around 20 years and has been a regular contributor to the on-line magazine Stereomojo.  There, he has been the resident computer audio guy and “value conscious audiophile” (aka “cheap bastard”).

John is a professor of analytical chemistry and a forensic chemistry consultant in his spare time, when he isn’t listening to music or evaluating gear. He tries to fit in plenty of time to hang out with his two teenage kids, his lovely wife, and the family cat, though only the cat also seems to harbor audiophile tendencies. John also enjoys running, cycling, golfing, hiking, or just about any other activity that sucks up time and money.

Read about John’s system here.