January 2026

Pure Power, Pure Pleasure — Pass Laboratories X600.8 Mono Amplifier Review
Quietly powerful, exquisitely precise sound that delivers recording-studio clarity.
Review By Tom Lyle

  The Pass Laboratories X600.8 monoblocks did not simply arrive — they announced themselves. Two Pass Laboratories X600.8 monoblocks for review here at Enjoy the Music.com, each a 123-pound monument of metal and circuitry, crossed my threshold and turned the upstairs listening room into a battleground for gravity. Their bulk filled the doorway, their presence and proportions, and for a moment the house felt like it had been claimed by something far more serious than furniture. Furthermore, I needed a little help getting them into position within my listening room. Besides being quite heavy and quite large (19" x 21.5" x 11" WxDxH), my listening room is on the top floor of our home.

Nirvana
On their website, Pass Labs says that the Pass Laboratories X600s can "...effortlessly drive the most demanding loudspeakers to musical nirvana." With 600 Watts under the hood, it should be obvious that these monoblocks can drive just about any pair of speakers one can think of, including my large reference speakers, but whether one's speakers can reach a state of bliss is subjective. Still, 600 Watts is quite a bit of power. Whether you'll need this amount of power depends on several factors, including the speakers' size and sensitivity, the listening room's size, and the kinds of music the listener will play through their speakers.

Loudspeakers
Less than a year ago, I took delivery of my new speakers: a pair of Raidho TD4.2 floorstanding loudspeakers (reviewed here). So, I was very happy when these new Pass Labs monoblocks were set up in my listening room. The Raidho speakers are more than 5' tall, 2' deep, and have seven drivers, including two 5" midrange drivers, four 6.5" bass drivers, and a ribbon tweeter.

Pass Labs was also the maker of the amplifiers I used with my previous reference loudspeakers, a pair of Sound Labs Majestic 545 electrostatics (reviewed here). Despite their size, they were not that difficult to drive. I used a Pass Labs X250.8 rated at 250 Watts per channel (Wpc) as reviewed at this link. I still had my older Pass Labs amp, a 350 Wpc X350.5, in-house. The latter amplifier with 100 Watts more power sounded best with the Raidho loudspeakers, even though it was an earlier Pass Labs model. But for these speakers to reach their full potential, I believed that more powerful amps would be helpful. I was delighted when these 600-Watt monoblocks showed up!

Feelings
Even though Pass Laboratories made my last two reference amplifiers, I have no problem putting my personal feelings aside when it comes to the brand of equipment I'm reviewing. I've been reviewing high-end audio equipment for 35 years, and many types of amplifiers have passed through my reference system, some remaining in it longer than others. I have no allegiance to any amplifier brand when I'm writing reviews.

Despite this, I admit there is at least one reason why I like Pass Laboratories' products: Nelson Pass. Nelson Pass is the founder and owner, and served as its chief designer when it was founded, and still designs many of its amplifiers. If you've heard Mr. Pass speak or read some of his papers, it's evident that he's intensely passionate about designing and building power amplifiers. It's a safe bet that he is designing and building a power amplifier as you read this review. I'm convinced that even if Nelson Pass weren't making his living manufacturing power amplifiers and similar products, he'd still design and build them.

White Paper
Before I started writing this review, I was lucky enough to acquire what we in the audio world call a "white paper" about the X600.8 and Pass Labs' .8 series in general. A white paper provides the reader with more in-depth information than promotional material Within this white paper, I found that Nelson Pass and I agree on the validity of measurements. Nelson Pass says that much of the .8 series, including the X600.8, wasn't built "objectively" in light of his feelings about the measurements. I think it is best if you read this short excerpt from Nelson Pass regarding his thoughts on these measurements:

"For many years, there has been considerable faith that if we simply keep improving the measurements of components such as amplifiers, then they would sound better. Initially, this was the case – older equipment was objectively flawed, and therefore, better measurements aligned with the listener's subjective experience. There's no doubt that some real progress resulted from these efforts, and now one can purchase some audio products at reasonable prices that measure far better than the old stuff. However, the disconnect between the customer's perceptions and the measurements persisted. Eventually, there were cases of state-of-the-art equipment that failed to sell because people didn't like the way it sounded."

Nelson Pass feels strongly that the Pass Labs .8 series is the best amplifier Pass Laboratories has ever designed and built. Much of the amplifier's innards were designed from lessons learned from their top-of-the-line Class A "XS" series. Mr. Pass also mentions that he likens the Pass Labs .8 series to the "flea Watt" amplifiers that he built when he was young. He says that the sonic qualities of these little amps can be found in their "Point 8" series, but with much more power and other superior attributes that come with these larger amps.

Nelson Pass says he follows a "simple" formula: more hardware for more power, fewer stages, lower distortion, and less feedback. But his secret formula also includes adjusting each amp individually before it leaves the factory, insisting that this will ensure each amp "operates in harmony that delivers the music experience". Nelson Pass "doesn't think audiophiles want technical perfection. They want to be happy."

Familiar
Readers might be familiar with my home stereo system that includes the Raidho TD 4.2 speakers, with front-end consisting of a Basis Audio Model V turntable, modified from a Gold model, which now features an AC synchronous motor. Because of the turntable's AC motor, I was able to use an outboard power supply and speed controller. A Lyra Atlas Lambda phono cartridge was mounted on a Tri-Planar 6 tonearm, with the tonearm's integral interconnects connected to a Pass Labs two-chassis XP-27 phono preamplifier (reviewed here). The phono preamp was connected to a Nagra Classic Preamp linestage (reviewed here).

The digital front-end of the review system included a computer-based music server with its USB output connected to the USB input on the rear panel of a Simaudio Moon 641 DAC/Network player (which earned Enjoy The Music's Best Of 2024 Award). The music streaming apps using true lossless audio Tidal and Qobuz were installed on my computer so I could play music from those streaming services. Still, I could also play music stored on several NAS hard drives. In addition to the apps on the music server's screen, these apps were also on my Apple iPhone and iPad, allowing me to stream music through Simaudio's MIND app, which lets one stream just about any type of digital signal through the Moon 641 circuits.

The 641 also had a jack for an Ethernet cable on its rear panel. An Ethernet connection allows one to listen to internet radio and streaming services and, via a home network, to listen to the music on my NAS hard drives, which store many terabytes of FLAC files with resolutions from CD quality 16-bit/44.1kHz all the way to DSD 64.

Help
I hope I can help some resist the urge to skip to the last paragraph of this review to read its conclusion with this short spoiler alert: The Pass Laboratories X600.8 monoblock power amplifiers are the best power amps I've ever heard from Pass Labs, and might even be the best amplifier I've ever heard in my system, regardless of make or model. Although without directly comparing other top-flight amps that have passed through my system over the years to the x600.8 monoblocks, I will hold off declaring this, especially since I have not yet heard one of Pass Laboratories' amps from their "XS" series, particularly their $93,000 XS300 300 Wpc Class A two-chassis monoblocks.

But today, when listening to the x600.8 monoblocks in my system, I often found myself either smiling ear to ear when the music was playing or remaining introspective and transfixed on the music that was playing.

The Pass Laboratories X600.8 monoblocks were such a transparent and dynamic pair of amplifiers that they made every record or file's signal sound as if I had a direct connection to the source. It also meant I noticed the sonic personalities, or lack thereof, of each component in the reference audio chain and how each affected the music. Thankfully, I'm lucky enough to have a decent system, so this was most often a positive.

Having the Pass Labs X600.8 monoblocks in my system was also a case of "be careful what you wish for." Because of the X600.8's exceptional sound quality, I feared I would have to re-listen to every one of my many favorite recordings to hear what they really sounded like! I would have to revisit not only every selection in my extensive record collection, but also every one of my favorite pieces of music available through the streaming services and the FLAC files on my hard drives.

Love
I love the" Living Stereo" LPs that were recorded meticulously by the RCA engineers during the early days of stereo in the mid to late 1950s. RCA had something to prove, since these records were made when mono recordings were still the norm. Two of my favorites Living Stereo recordings are versions of Bartok's two most popular pieces, both conducted by the same maestro and performed by the same orchestra. Both were composed by Bela Bartok, conducted by Fritz Reiner, and played by the Chicago Symphony Orchestra (CSO). The first was Bartok's Concerto for Orchestra. Even though Concerto For Orchestra is top-notch, lately I've been spinning the other Bartok piece in this series, his Music for Strings, Percussion and Celesta.

I had a very nice original pressing of Concerto For Orchestra, but in 1994, while it seemed like everyone else was buying CDs at Tower Records, some devoted vinyl fans like me scored near-perfect heavyweight reissues of Bartok's pieces and other titles from Classic Records. Classic Records pressed these records with the same mastering equipment used when they were first pressed, but updated to conform to modern specifications.

No matter how good a two-channel high-end audio system is, reproducing the sound of musicians performing on a stage with a 50-foot proscenium isn't realistically possible. But the X600.8 monoblocks captured what I often call the "gestalt" of this orchestra. What I heard when spinning this fantastic vinyl through the X600.8 monoblocks nearly made me fall off my listening seat. I felt as though I were hovering above the conductor's podium, hearing each instrument and group of instruments as if I were sonically viewing them. The recording was made in the 1950s, but it sounded as if it were made yesterday.

The separation between instruments and sounds on both recordings was unparalleled in my listening room. Even the tape hiss on these vintage stereo recordings was separated from the music, floating above it. The tape hiss was very easy to ignore, because even though I've heard these pieces many times before, playing it through the Pass Laboratories X600.8 monoblocks and then through the speakers was so engaging.

Adding to all the attractive audiophile traits these Pass Labs amps possessed, one of their greatest attributes was a vast, drawn-to-scale soundstage. The x600.8 monoblocks rendered Bartok's tone poems with a drawn-to-scale soundstage that filled the space between my speakers, making the string sections and winds sound as if they were coming through a center-channel speaker.

The percussion on both Bartok pieces sounded as if it were emanating from behind my front wall, as the rest of the orchestra wrapped me in the CSO's lush strings. On the right side of the orchestra, the basses and cellos produced low-end frequencies that sounded very realistic. The windows in my listening room rattled, as did my guts when the large concert bass drum was whacked in Bartók's Music for Strings, Percussion and Celesta, mainly during the first, second, and fourth movements.

During the audition period here at Enjoy the Music.com, the Pass Laboratories X600.8 monoblocks sounded very transparent, especially when playing well-recorded music. But I am as much a sucker as the next guy, because just because these amplifiers might or might not have ideal measurements isn't the issue, because to my ears, what I heard is a pair of amplifiers that were up there with the most transparent amps I've ever had in my system.

The Pass Laboratories X600.8 monoblocks were transparent-sounding to me because of what I stated above. The amps let me hear way to the back of the concert hall. In both pieces, there is a section where the percussionist is playing a solo during a point where the rest of the orchestra is silent: a snare drum in Concerto For Orchestra during the second movement, and a xylophone in Music for Strings, Percussion and Celesta when the third movement begins. When these solo percussion parts occur, I imagine the stage's dimensions as the sound reverberates off the walls in that part of the hall.

The Pass Labs X600.8 monoblock amplifiers' transparency was also evident in its extremely black background, to the point that I could not tell whether my linestage was on mute or set to a realistic volume when no music was playing. The less noise there is, the more details of the music are heard. Even the slightest level of noise can subconsciously be distracting, so one might think that an amp that measures well in this area would be better than one that measures poorly. This is a moot point when it comes to how the music makes one feel. If a component sounds transparent, it is transparent.

When playing either of the prized LPs, these monoblocks' transparency yielded a phenomenal-sounding midrange, which I noticed more on Music for String Percussion and Celesta, perhaps because a smaller ensemble was used for this piece. Fewer instruments might have masked the sound, but this sound took on the transparency of the Pass Labs monoblocks, resulting in a gorgeous sound.

Musical
Just because I used these two Bartok pieces as musical examples doesn't mean the Pass Laboratories monoblocks could only reveal their best sound when amplifying classical music. Those who know me should remember that there are only two types of music, good and bad.

While the Pass Labs X600.8 monoblocks were in my system, I started, as usual, by playing a Kraftwerk track digitally through the Moon 641 network player/DAC. I've also been known to evaluate equipment and listen to hard rock and metal, all types of rock, and plenty of jazz. These Pass Labs monoblocks did not favor any genre of music over any other. As they amplified the signal of any genre of music that passed through them, they provided the muscle and/or the appropriate delicacy.

Positive
I could have easily made this review on Enjoy the Music.com twice as long if I had described every positive sonic characteristic of the Pass Laboratories X600.8 monoblock, as it is nearly ideal-sounding. I'm saying "nearly ideal" to avoid painting myself into a corner, since there is at least a slight chance that I might one day be lucky enough to review a Pass Labs amplifier higher up in Pass's line, specifically one of their XS series amplifiers!

The large Pass Labs X600.8 amplifiers also looked very impressive sitting on their amp stands in front of my equipment rack. Their cool blue, large, illuminated meters on their aluminum front panel, and their large black heat sinks, which took up all the room on their side panels, gave the amps a high-tech, industrial-chic look.

At $31,460 for the pair, the Pass Laboratories X600.8 monoblocks aren't bargain-priced. Nor are they the most expensive monoblocks Pass Labs offers. Simply because the X600.8 monoblocks are worth their asking price doesn't mean much if one cannot afford them. Still, readers of Enjoy the Music.com realize that audiophiles can get quite crafty when it comes to raising funds, especially when we become enamored of a component and wish to purchase it. Highly recommended? You bet!

Specifications
Type: Solid-state monoblock audio amplifier
Frequency Response: 1.5 Hz to 100 kHz
Power Output: 600 Watts @ 8 Ohms (1200 W @ 4 Ohms)
Operating: Class AB
Gain: 26dB
Distortion: 1% @ 1 kHz, full power
Input Impedance: 50 Ohm via RCA and 100 via XLR
Damping Factor: 200
Dimension: 19" x 21.5" x 11" (WxDxH)
Weight: 123 pounds
Price: $31,460 pair

Manufacturer
Pass Laboratories
13395 Airport Road
Suite G
Auburn, CA 95602

Voice: (530) 878-5350
Fax: (530) 878-5358
Email: info@passlabs.com  
Website: PassLabs.com