The following quote has been incorrectly attributed to Nelson Pass, founder of Pass Laboratories: “Listening to my amplifiers is like listening to tubes, but without the hassle.” Someone else may have said it, but Pass never did and has, in turn, commented about the quote, “While I strive to capture some of the qualities that the best tube gear has, I don’t feel that my sonic goal is really the same, nor would I want it to be.” For my own part, as an erstwhile tube amp enthusiast for the past 20 years and having had the pleasure of recently spending a few months with a Pass Labs amp, I can verify that much of what Mr. Pass said about capturing some of the qualities of the best tube gear rings true. The solid-state X350.5 does, indeed, sound similar to a fine tube amp, both in its general tonal character and in the way it “tracks” musical phrases (more on this later). However, the X350.5 offers much more than tube-like affectations.
First of all, the amp has massive power reserves and can drive just about any speaker with ease. It is rated at 350W per channel into 8 ohms and 700W into 4 ohms—indicative of a very stout power supply and robust output stage. The X350.5 also operates in Class A for its first 40 watts, and Class A amps tend to sound much more powerful than typical Class A/B or Class B amps of the same power rating. As a result, the X350.5 drove the somewhat power-hungry YG Kipod II Signature Passive speaker with absolute authority, imparting a general feeling of ease in demanding music passages, which never became harsh or otherwise exhibited strain—often a sign of a stable and robust amplifier. Tube power amplifiers with this sort of power output and speaker control are usually quite expensive. At $11,550, the X350.5 is certainly price-competitive.
OK, so I estimate the Pass Labs X350.5 represents a very good value both in its tube-like qualities and its very robust power output, but what about its other sonic characteristics? The amp is “liquid” in its ability to sound grain-free and non-electronic, and is “compelling” in its ability to reveal musically interesting nuances in the source material. Since much of the music I listened to with the X350.5 was likely being handled via its first 40 watts, I believe this smoothness and nuance can be attributed to the Class A effect. (The pure Class A Esoteric A-03 power amp also has this smooth, continuous quality— supposedly the result of Class A’s elimination of crossover-notch distortion.) Whatever the reason, music tends to take on a silkier, more relaxed quality through the X350.5 than what I usually hear from solid-state amps at this price level.
Unlike some solid-state Class A amplifiers, though, the X350.5 does not have a hint of that “buttery” sound that can make music sound a bit too smooth due to a slight blunting of transient leading edges. It also never veers toward the syrupy or the euphonic, like certain tube amplifiers do. The X350.5 simply presents music in a non-fatiguing, low-noise way, leaving the listener with a highly musical and engaging experience.
It is this particular aspect of the X350.5’s performance—its uncanny way of drawing the listener into the recorded musical performances—that I find so compelling. It does this so well that it almost made it difficult for me to take my usual analytical approach to this review. For instance, during one late night session in which I had intended to spot-check a few familiar cuts for various tell-tale sonic characteristics, I ended up instead listening to several tracks all the way through just for the sheer pleasure of it. I finished the session with the Poulenc Concerto in G minor for Organ, Strings and Tympani [Erato] on LP and was struck by the solid foundation of the organ notes and by the wide sweep of the mood changes as tempo and dynamic intensities shifted over the arc of the concerto. Mind you, the X350.5 does not favor a particular kind of music over others: large and small classical works, driving rock, intimate jazz, and that audiophile favorite, female vocals, are all served well.
The tonal balance of the X350.5 is basically neutral, with a subtle and pleasant richness in the midbass and just a hair more softness in the upper frequencies than I am used to from solid-state. In this way, the comparison to some push-pull tube amps is not too far off the mark. Where the comparison fails is in the definition and control the X350.5 brings to the bass, qualities that can sometimes be a bit lacking in tube amps. That bass performance—that fantastic clarity, heft, and grip—just makes for a more “real”-sounding and satisfying experience. For me, the X350.5’s neutral tonal balance, solid bass foundation, and liquidity, taken together, tended to shift my focus to the musical “mood” of the performances rather than to various audiophile criteria like imaging, hall sound, air, and so forth. The particular way the X350.5 seemed to emphasize the musical “ebb and flow” actually reminded me of how I respond to live music and, to some extent, to certain fine tube amps.