One of the most venerable designers of solid-state amplifiers over the past 40 years is the legendary Nelson Pass. He has over seven patents that cover some of his most creative and groundbreaking circuit designs. From the very beginning, all of Pass’ amplifier designs were based on Class A topographies, which offer the rich color and the overall liquidity of what tubes have to offer with the grunt, power, and control of the best solid-state amplification. Over the years, I have owned his Threshold Stasis 2 amplifier, Threshold SA-1 mono blocks, Pass Labs Aleph 1 mono blocks, Pass Labs X350.5 amplifier, Pass Labs XA100 mono blocks, and most recently Pass Labs XA60.5 mono blocks. Besides totally enjoying how each amplifier produced beautiful music, I also appreciated each amp’s industrial build quality and rock-solid dependability, never needing repair.
Plus, I have great respect for two aspects of Mr. Nelson’s design efforts. First, each new generation of his amplifiers offers significant improvement over the previous generation, which was already great in its own right. Secondly, unlike many other manufacturers who come out with new versions of their amplifiers during each buying cycle to maintain sales, Nelson Pass and his team only bring a new generation of amplifiers to the market when they believe they have learned how to make a more musically satisfying amplifier.
I have had a pair of Pass Labs XA60.5 mono blocks for over five years, and it took seven years for Pass Labs to come out with the new XA.8 series. The series includes a total of 10 new amplifiers in two-channel and mono-block varieties. I chose to review the XA60.8 mono blocks, which retail for $12,800 per pair, because of my familiarity with the XA.5 version. Numerous and major changes were made to the new XA.8 amplifiers, such as output stages that bias more deeply into Class A operating region, lower distortion rates, higher power MOSFETS, much larger power supplies, and much more extensive and larger heat-sinking on each mono block. During my audition of the XA60.8 mono blocks, I discovered that they run much cooler than my XA60.5s because of their more efficient heat-sinking.
Each of my XA60.5s weighs 62 pounds, while the XA60.8 weighs 88 pounds due to its heat-sinking and a much larger and heavier power supply. It measures 19 inches long by 21.25 inches deep by nine inches high. A large bias meter in the center of the matte-silver front plate glows blue when the amplifier is on; the needle within the meter only moves if the amplifier leaves Class A. The on/standby button is located below the bias meter. On the back is a master power switch, a pair of handles, a pair of single-ended (RCA) inputs, a pair of balanced (XLR) inputs, the IEC power input, and finally a pair of speaker-wire terminals. The speaker-wire terminals were the best I have ever encountered on an amplifier, composed of large wings that allow you to hand tighten your speaker wire spades until you hear a click, which indicates that the connection is secure. Unlike the upscale appearance of the XA60.5, the XA60.8 is rather austere and understated in its overall look. The casework and overall quality of the chassis are superlative, which is what you would expect from Pass Labs.
The XA60.8 mono blocks are rated at 60 watts pure Class A into eight ohms and 120 watts pure Class A into four ohms. The XA60.8 leaves pure Class A after 122 peak watts and moves into Class AB for tremendous headroom on very large-decibel peaks. Regardless of which speakers I connected or at what extreme sound pressure levels I listened, I could never get the XA60.8s out of their Class A biasing range.
To provide first-class packaging and to ensure safety and protection of the amplifiers during shipping, Pass Labs used extremely thick cardboard boxes for the exterior as well as the interior, along with component-fitted pieces made of high-density foam inserts to protect the amps from rough handling. Although the demo amplifiers I received had enough hours on them to be considered fully burnt-in, I still put another 50 hours on them before I began my serious auditioning process. My auditioning system was composed of the Aerial Acoustics 6T tower speakers and the Lawrence Audio Cello tower speakers, as well as the Backert Labs Rhythm 1.1 preamplifier (review forthcoming). The source was a reference MBL 1621 transport driving the Concert Fidelity -040 battery-powered hybrid DAC (review also forthcoming). All wiring was silver reference MG Cable ICs and MG Cable reference three copper ribbon speaker wire. The entire system was powered by Harmonix Studio master power cords and a Running Springs Audio Dmitri power conditioner.
It was very apparent to me from the beginning of my listening session that this new generation of XA.8 amplifier was an across-the-board improvement over my wonderful-sounding XA60.5. One of the most important recordings I use when reviewing a new piece of equipment is the late, great saxophonist Johnny Griffin’s “The Kerry Dancers and other swinging-folk music” (XRCD Riverside). I was fortunate enough to hear him play on many occasions. Therefore, I have a very good take of what his style, tonality, and timbres were in real life. The XA60.8s were the quietest high-power amplifiers I have ever had in my system. The already very low noise floor of the XA60.5s was surpassed by the utter lack of background noise by the XA60.8s. What this meant musically was that the smallest micro-details were more easily heard and any ambient cues that revealed where the music was recorded were more apparent. The tonality, overall color, and timbres of Griffin’s saxophone were produced in a totally natural and delicate way and came very close to what I remember him sounding like in his live performances.
The next area that struck me as an improvement over the XA60.5 was how the XA60.8 produced a much more powerful and accurate rendering of the lower midrange, which gave an even more solid foundation to orchestral music to help it sound more lifelike. When I played Erich Kunzel’s Orchestral Spectaculars (Telarc) — particularly, Rimsky-Korsakov’s “Snow Madden-Dance of the Tumblers” — the orchestra’s sense of weight and authority was more realistic than it was with the XA60.5. This new authority in the lower midrange was integrated very well with the rest of the music, producing an octave-to-octave seamlessness that was very compelling and natural sounding.
The new album by 3B Hammond organist Larry Golding, called Ramshackle Serenade (Pirquet), is a great test of an amplifier’s ability to produce extended and naturally airy high-end frequencies. Drummer Bill Stewart is a master of using either sticks or brushes when playing his cymbals, and the Pass Labs XA60.8 mono blocks were quite up to this task. The air, bloom, and decays of Stewart’s cymbals were all rendered in a vivid manner.
Kenny Burrell’s classic album Midnight Blue (Blue Note) offers some of the most beautiful jazz hard-bop takes on the blues genre that have ever been recorded. It was recorded by the great Rudy Van Gelder to have a wonderfully rich and warm tonal perspective that allows you to relax and pick up the emotionality of the music. The XA60.5 was one of the best solid-state amplifiers at producing this experience of relaxing and relating to the emotional content of Burrell’s guitar playing. However, the XA60.8 offers even more liquidity, more density of tonal color, and a natural easiness to the overall presentation that allows you to feel closer to the music. Another improvement was the more precise layering in the soundstage.
Another selection that really showed off both the tremendous soundstaging and the transparency and resolution of the XA60.8 mono blocks was Carlos Santana’s classic album Abraxas (Sony). When the first notes emerged on the song “Singing Winds, Crying Beasts,” they filled my room with a wall-to-wall soundstage and a holographic, three-dimensional representation of the different instruments as they swirled left to right in the mix. I have listened to this great album hundreds of times over the years with very revealing amplifiers, and I thought there were no more micro-details or nuances left to be uncovered. However, I did hear new information, such as the musicians talking or individual guitar strings distorting, that the XA60.8s revealed where the other amplifiers left them out.
Finally, I wanted to see how the XA60.8s would reproduce the human voice, which is one of the hardest instruments to replicate. I chose Peter Gabriel’s album So (Geffen Records) because the sound quality of this studio recording is rich in its overall tonal color and allows you to hear his voice in a natural way. On the track “Don’t Give Up,” both Peter Gabriel’s and Laurie Anderson’s voices were pure in their tonality, and each singer’s unique timbres were very easy to hear because of the total clarity offered by the XA60.8s.
It was difficult to identify any shortcomings in the performance of the Pass Labs XA60.8 mono blocks. I am sure that you could find a speaker that would need even more watts and current to drive it successfully. That’s where the XA60.8’s bigger brothers come in, if needed. You also must be somewhat careful with what preamplifiers you use with these mono blocks. I tried a few highly regarded solid-state preamps (although none were Pass Lab’s own highly regarded preamps) and lost some of the sonic beauty compared with what I got with the excellent Backert Labs Rhythm 1.1 preamplifier. These amps are reference level in terms of clarity/transparency and will expose any shortcomings in upstream gear.
Comparison and Competition
Two amplifiers I have experience with that would be price competitors to the Pass Labs XA60.8 would be the Classé CT-M600 mono block that retails for $13,000/pair and the Parasound Halo JC 1 mono block that retails for $10,000/pair. The Classé offers great dynamics and transparency/micro-details but has a less accurate, drier tonal balance than the XA60.8. The Parasound offers better overall tonality than the Classé but is not as tonally pure as the Pass Labs. Another significant difference between the Parasound and Pass Labs XA60.8 is the latter’s level of transparency, which allows the nuances of the music to be effortlessly heard.
Because of its pure Class A wattage, the Pass Labs XA60.8 mono block has an advantage over other similarly priced Class AB designs in offering a very special overall musicality and beauty. However, to build an all-pure Class A amplifier means extra expense, if it’s done correctly, because you need more heat-sinking to dissipate the intense heat and very high-quality internal parts to withstand the stress of the output transistors never turning off. After getting used to the musical capabilities of a pure Class A design like the Pass Labs XA60.8 mono blocks, it can be quite difficult to go back to any good Class AB amplifier. The extra expense in building a pure Class A design is worth the up-charge for the sonic virtues it renders in its reproduction of music.
The Pass Labs XA60.8 mono-block amplifier carries on Nelson Pass’ long tradition of always improving upon the previous generation of amplifiers in order to achieve a more musical presentation. In all the important sonic categories — such as reduction of the noise floor, greater transparency, rich and gorgeous color/timbres, overall liquidity, size and accuracy of the soundstage, image density, macro-dynamics, and bass extension — the XA60.8 is an improvement over the previous XA60.5 mono block. I used to believe that I would be more than satisfied with whatever Nelson Pass amplifier I owned at the time; however, with his reputation to strive for greater excellence, he never fails to create a new generation of amplifiers that is even more superlative than the one before it. The XA60.8 is exceptionally built, offers the needed foundation in its overall power/dynamics, and also provides a musical delicacy and sweetness you normally only get from the finest tube-based amplifiers (without the hassles of maintaining them), bringing you emotionally closer to the music with great ease.
Nine months ago, I reviewed another pure Class A amplifier, the Musical Fidelity AMS50, and rightly gave it five stars for its stellar performance. The AMS50 also had the beautiful color, tonality, and image density of tubes and the pop, power, and control of what solid-state designs have on tap. I was very tempted to dig deep into my budget and purchase this wonderful amplifier. However, I came to the conclusion that my Pass Labs XA60.5 mono blocks were so close in their performance to the Musical Fidelity AMS50 that the hange wasn’t necessary. This time around, I not only knew that the Pass Labs XA60.8s significantly surpassed my XA60.5s in all ways, but that the XA60.8s were the finest solid-state amps I had ever heard in my system and, without hesitation, purchased the demo pair.
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