Review: Pass Labs X600.8 mono block Power Amplifier

Pass Labs X600.8

Do you want the hernia or the double hernia? The voice on the phone asking that question was Bryan Stanton, a communications specialist who works with Pass Labs.

A few weeks prior, we had begun chatting about Pass sending me one of its X-series amps for review. I had agreed, and now Pass wanted to know if it should ship the 127-pound X350.8 stereo amp or a pair of the top-of-the-line, 123-pounds-each X600.8 mono blocks. I didn’t hesitate for even a second. “Oh, the X600.8s. Definitely,” I told Stanton. Apparently, my lack of hesitation to go for the big mono amps was the source of some amusement around the Pass office, with the punch line being that reviewers are a rather predictably size-obsessed bunch. (Careful what might end up in an email thread, guys…)

I didn’t take it too personally. After all, there is some truth to that – quite a bit, actually. My particular fascination tends to be less about dimensions, however, and more about performance. I, like many audiophiles, always am curious about the state of the art. If a manufacturer has one product in the middle of its line that’s impressive, what does the flagship sound like? I might not be able to afford it, but I want to hear it nonetheless.

Pass, a pioneer in solid-state power, makes a mind-boggling number of amplifiers. I suspect the explanation for the company’s exhaustive catalog is that its founder, legendary designer Nelson Pass, is a man who is constantly tinkering and getting new ideas. In my mind, I imagine Pass as kind of the Neil Young of high-end audio. In the same way that Young, after decades working at his craft, continues to issue albums at a relentless pace, the industry-veteran Pass has a comparable and seemingly bottomless well of creativity. As a result, Pass has at least five separate lines of amps – with each containing many models – plus an entire second brand, First Watt, for his additional experiments.

123 pounds of American mono block.

The chiropractor-pleasing X600.8s I received are, at $26,000 USD a pair, the top of the X.8 high-voltage series. They are not, however, the ultimate expression of Pass Labs technology, as there stand the pure Class A Xs300 monos at $85,000 USD a pair. Still, the X600.8s are formidable in their own right. While many Class A/B amps operate in A for a handful of watts before switching to B, each X600.8 stays in room-heating Class A for the first 50 of its massive 600 watts of output (into eight ohms).

I’ve had plenty of high-current, high-watt amps in my home. For many years, my reference amps were either a Krell KSA-150 (150 watts, all Class A) or a Krell FPB 300 (offering 300 WPC, although with a sophisticated A/B switching circuit). I remember when I first hoisted the Dan D’Agostino-designed FPB 300 out of the box (with a lot of help) and connected it, I was afraid to push the “on” button. The thought of unleashing so much power was a bit frightening. That feeling came back as I set up the X600.8s. First of all, Stanton was right about the double hernia. The heavily muscled Fed Ex man sighed loudly and gave me an eye-roll when I asked if he would mind hauling the boxes through my front door. Grudgingly, he carried them about 3 feet and plopped them down in the hallway. There they sat for a while, mocking me, until I talked a pair of home handymen (helping with a remodeling project) into moving them into the listening room. Standing clear and watching their faces as they wrangled these amps was a little hard on my ego, I have to admit. Still, discretion is the better part of valor, and all that. They’d saved me from injuring myself, or so I thought…

The X600.8s are handsome amplifiers. The casework is gorgeous, with a tall, polished-stainless face-plate sporting a huge, circular VU meter illuminated by a blue glow, underneath those is an on/off button. On the back, the amps have two sets of output binding posts. The arrangement is a little unusual, with each pair (positive and negative) arranged vertically on one side. The posts are well-marked, however, and are extremely robust. Near the top is a single RCA input, and below that, an XLR jack for balanced operation. The XLR plugs come with a shorting pin that should be left in if the RCA connection is chosen.

I disconnected my Merrill Audio Thor mono blocks and inserted the X600.8s using two-meter XLR runs of AudioQuest Lapis. Next, I attached my Transparent Reference XL speaker cable, which was a little difficult with the vertical location of the outputs and the thick, stiff wire. Finally, I plugged the heavy, supplied power cords into my new PS Audio PowerPort wall receptacles which I added when I had three 20-amp dedicated lines installed during remodelling. The X600.8s now were fully integrated into my reference system, which also included a Mark Levinson 380s preamp, Musical Fidelity M1 CD transport, Mark Levinson 30.5 DAC and Revel Studio loudspeakers. Other wire included AudioQuest Sky XLR from the DAC to preamp, as well as Kimber/Illuminati Orchid and MIT Reference digital cables. My turntable was out for modification, so I listened only to CDs.


As I sat on the couch for a few minutes, still somewhat intimidated to flip the switches on the Pass Labs behemoths, my wife came in to check on me. “Oh my God, you’re bleeding,” she said, looking at a blood-soaked white sock on my left foot. Apparently, I had accidentally brushed up against the X600.8s, which have dozens of sharp heat fins on their sides. The brand-new carpet now had a blood trail. Word to the wise: Either wear boots during set-up or try not to be as clumsy as yours truly.

I decided to kick-start the new amps with a raucous Keith Richards solo track, “As Wicked as It Seems.” As I cautiously brought the volume up, I forced my eyes back open and slowly looked around. The lights didn’t dim, my speakers weren’t melting and I hadn’t succumbed like a character targeted for a dirty deed by AC/DC. (High voltage!) So far, so good. I switched to more ethereal fare, Miles Davis’ In a Silent Way, for an extended repeat-play period that lasted about a week. As I waited for the amps to burn in, I read up on what Nelson Pass was trying to accomplish here.

The Class A/B line of the Pass Point-8 Series, which consists of the X600.8 and four lesser-power amps, is the product of seven years of research. The new amps replaced the well-regarded Point-5 series. In the owner’s manual for the X600.8, Nelson Pass writes that his goal was to advance the key elements of the older amps to the next level. This wound up involving more of everything. The X600.8 uses bigger hardware that is biased more deeply into the Class A operating region. The front-end circuit is more sophisticated and customized for each model. The power supply also is beefier. And, finally, there are those heat fins – larger and more abundant to carry off the extra heat. The solid-state front end uses a mix of JFET, MOSFET and bipolar devices made by Toshiba. The parts actually have been discontinued, but Pass believes so strongly they are superior to anything else that he ordered a deep stock to allow both construction of new amps and repairs. “The result is a front end with high stability, low distortion and (low) noise,” Pass says. “It has a very high input and is DC-coupled. There are no compensation capacitors — in fact, there are no capacitors in the amplifier circuit except across the shunt bias regulators and the power supply.”



Talk Is Cheap.

Since I’d blown the packing dust off with Keith Richards, it seemed appropriate to return to the same number to start my formal listening. “As Wicked as It Seems,” from Keef’s 1992 Talk Is Cheap LP, could easily have been a great Rolling Stones single. Here, Richards’ shaky, nicotine-ravaged vocals are ducked down in the mix. The X600.8 perfectly revealed the wisdom of that production decision. His five-string rhythm guitar had its trademark bite and swagger, however, matched by the equally crunchy lead work of LA session godhead Waddy Wachtel. Several other things immediately stood out about playback of the song through the Pass Labs X600.8s. First, there was an effortless quality to the sound, even when Richards’ band, the X-Pensive Winos, shifted to full tilt. Second, there was a weight and solidity to the presentation that was combined with an unusual sense of depth.

The Trinity Sessions.

I switched to the Cowboy Junkies’ slow, hushed cover of Lou Reed’s “Sweet Jane” from their chill-out classic The Trinity Session. The song was recorded live to RDAT in a Toronto church, with the four players circling a single Calrec Ambisonic microphone. This spare 1987 song contains just bass, electric guitar, drums and vocal. On an exceptional system, however, the ambiance of the old venue is a key fifth element. The X600.8s recovered this ethereal quality to a remarkable degree, with Michael Timmins’ guitar echoing off the walls and sister Margo Timmins practically whispering the lyrics into my ear. Probably because of the straight-to-tape nature of the recording, the vocals have some occasional sibilant issues. On the X600.8s, those sibilants were still there, but the lush midrange I attribute to the first 50 watts of Class-A power seemed to minimize them. At the same time, both ends of the frequency extreme were equally as impressive, with Alan Anton’s bass having both depth and tunefulness and Peter Timmins’ subtle cymbal work portrayed with a silken sheen.

Another recording that really highlighted the X600.8s resolution and dynamic capabilities was jazz singer Lyn Stanley’s The Moonlight Sessions, Vol. 2. The second song, a cover of the American Songbook standard “The Very Thought of You,” begins with Stanley introducing the lyric over a tasteful piano line. Then bass enters, following by the swell of an orchestra. The X600.8s easily handled the dynamic shift, with the song getting louder but not aggressive. Stanley’s voice, which was recorded on Frank Sinatra’s old Neuman tube mic, had notable texture and the piano – a difficult instrument to capture – sparkled.

Plugged in, and turned on.

Occasionally in reviews, I will rave about how a component created reverb trails that stretched far into the back of the studio. The X600.8s had that kind of sound-staging, but they did more than that. Track after track, each player’s instrument was extremely full-bodied. It was easy to conjure the illusion of real people gathered in a room. The best way to describe it would be that the experience was the aural equivalent of witnessing live actors in a play, rather than watching the same production on a movie screen. One has people occupying real space on a large stage, while the other is a projection that doesn’t extend behind a thin screen.

In comparison to my reference Merrill Thor mono blocks, the X600.8s had better-defined lower bass, more weight and a fuller midrange. The perceived dynamic range seemed wider, and the Pass amps had the uncanny ability to portray three-dimensional players in space. The 200-watt Class D Thors, though, had a slight edge in overall smoothness. Finally, the X600.8s got much hotter than the Thors. If you have a cat, it won’t curl up for long on top of these babies. My Krell units, meanwhile, had superior bass slam, but showed their age in nearly every other category. To be honest, none of these matchups was a fair fight, since the most expensive of the Krell amps had half the watts and retailed for $10,000 USD when new, while the Thors sell for $4,800 USD and come second in the Merrill line to the $12,400 USD Veritas monoblocks.

After swapping amps, I decided to see how the X600.8s would fare with speakers that are significantly different from my Revel Studios. I inserted a pair of MartinLogan Prodigy hybrid electrostatics, which have a nasty impedance dip that goes down to about 2 ohms. The Pass monoblocks’ significant Class A power brought a new depth to the Prodigy’s magic midrange and both extended and polished the high frequencies.

Full size, full weight: X600.8


At $26,000 USD a pair, the Pass Labs X600.8s are the most expensive amplifiers I’ve ever had in my reference system. Heck, they are by far the costliest component of any kind I’ve ever unboxed. Yet, these unabashedly heavyweight powerhouses offered a fascinating opportunity to see what a gifted, veteran designer could achieve with fewer cost restrictions. Perhaps the outstanding attribute of the X600.8s was their ability to portray musicians and their instruments as 3-D, full-size entities, realistically positioned left-to-right and front-to-back across the soundstage. I’ve heard some other listeners describe the Pass house sound as “solid state with warmth.” After a months-long demo, I think what they’re hearing isn’t really warmth, which to me would imply some degree of reduction in texture, detail and resolution. Instead, I suspect what they’re reacting to is a remarkable, full-bodied presentation. Pass amps – particularly the X600.8 – offer solidity without excessive thickness, detail without brightness, and dynamics without fatigue. Add to that stunning depth with realistic instrument layering, and you have quite a feat of engineering.

Simply put, the Pass Labs X600.8s are the finest amplifiers I’ve ever had in my home, and are on the short list of the best I’ve ever auditioned anywhere at any price. For those who can seriously shop at this level, they should be a must-audition. For everyone else, these mono blocks still deserve to be heard, if for no other reason than to attune your ears. Audiophiles in the latter category, if it’s any consolation, at least you won’t have to risk the double hernia getting them home.

Associated equipment:

  • Preamplifiers: Pass X12, Mark Levinson No. 380s
  • Amplifiers: Pass Labs X600.8 monoblocks, Merrill Audio Thor mono blocks, Krell FPB 300.
  • Digital: Mark Levinson No. 30.5 DAC, Musical Fidelity M1 transport.
  • Loudspeakers: Revel Studio, Martin Logan Prodigy.
  • Cable: Transparent Reference XL speaker cable, AudioQuest Sky and Lapis XLR interconnects, Kimber/Illuminati Orchid digital, MIT Reference digital.
Editor’s Choice recipient: The Pass Labs X600.8 power amplifier.


About John Stancavage 196 Articles
Contributing Editor for Part-Time Audiophile


  1. Nelson Pass è uno dei più grandi nel panorama audio, detto questo mi chiedo perché i cavi blu e rossi che portan corrente ai stadi finali son cablati in modo non molto corretto.

  2. Wow, very nice. I don’t think I’d ever buy them. Not just because my pockets aren’t anywhere near deep enough but because at what point in audio fidelity is the point of diminishing returns?
    Don’t get me wrong, I’m a music junkie as is anyone with a soul. I’m also a want a be hardware junkie. I buy the best I can afford. Usually used components. But, would I realy be able to tell the difference between a fabulous system and one that is priced at ludacrice level? I’m not so sure.

    • Yes of course you can. Don’t be hard on your ears just because of price. Try What’s Best Forum to read discussion of the differences. They’re obvious and worthy. Of course personal taste, room acoustics and everything else matters. No need to close a door to your mind that your pocket and 98% of everyone else can’t afford. Lol just enjoy great engineering and design wherever you find it.

  3. Dear Peanut Gallery; If you aint heard these amps and cant afford them and you cant post a well-mannered comment, stuff a smelly sock in your ____ “opinions”, please. And thank you. Sincerely, a poor shmuck who aint heard nor can afford these things. XO and tootles!!!!

  4. For $26K, then why wouldn’t this guy include the best AC line cord to make it the best $26K, think about the markup!!! What changes with a “different” AC cord, that doesn’t happen with a “standard” line cord, so you are saying Pass, really doesn’t know everything? Or did he FORGET, as is mentioned in the article ..PROFIT on each dopey unit

  5. “Wicked As It Seems’ is from Main Offender, not Talk Is Cheap. Good song though.

  6. $26K!!! That is funny. And that means they shouldn’t have sharp edges on the heat sinks. It also means it’s poor build quality. So they buy generic heat sinks, unfinished, and charge $26K, are you a rube? Audio by VanAlstine has better stuff more potent for much much much less, checkout his new MONO 800+ watt amps…ya can buy 4 of em for less than these Pass crap, Pass on Pass, fancy box, that is not even well finished.

  7. Nice review. I’ve read in other Pass Labs reviews that he ships with a stock power cable as you used. Reviewers who used upgraded power cables such as MIT Cables report another significant uptick in performance. Imagine that…😉

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