Pass Labs XA160 and X600.5 Monoblock Power Amplifiers

A Tale of Two Amplifiers

This is not a review for audiophiles who have blundered into the wrong magazine and think that all amplifiers sound the same. It is an exploration of two new amplifiers from the same designer and firm, of how their sound differs in nuance, and how they differ in terms of their interface with different speakers. It also is in some ways a warning about amplifier reviews and system interfaces, and about the need to carefully listen to the synergy—or lack of it—between your power amplifiers, speaker cables, speakers, and listening rooms.

I also should stress that the two amplifiers involved—the Pass XA160 and X600.5—do sound very much alike. They should. They are both made by Pass Audio Labs; they are both designed by teams led by Nelson Pass; they are built on the same chassis; they both have the same basic “super symmetry” and two-gainstage circuit topology. They also are both expensive high-end products where cost is a minor constraint on performance; both sell for $18,000 the pair.

Both designs are based on long evolutionary experience. Nelson Pass is one of the most famous amplifier designers in the high end, and the design teams he has led both at Threshold and at Pass Labs have consistently pursued accuracy and sonic purity, not gimmicks or fashion. Like most of the best high-end designers, Pass has gotten steadily better. Each generation of amplifiers he has produced has been a bit cleaner, has better low-level transients and dynamics, and is sweeter and more detailed. He has also been consistent in the way he “voices” his amplifiers: open and detailed, not warm and forgiving; extended highs and flat levels of upper midrange energy; equally flat mid and upper bass, with no gimmicks to give the sound more punch and “rhythm.”

Like most audiophiles, I’m not willing to make one more compromise than I have to. I want both power and nuance. I want an amplifier that can drive virtually any speaker, regardless of character and load. I want it to sound exactly the same every time I turn it on, so I can be sure that I hear the real differences between the components I’m reviewing in my reference system. I also want it to be both neutral and “musical” in the sense that it is revealing and does not color or exaggerate, but also is not “analytic” or fatiguing.

Pass Labs has delivered what I personally want in one of my reference components ever since it introduced its Aleph series. I have paid close attention to the Pass X-series ever since, and when the series of events that led to this review began, I was using the Pass X600. Shortly after the XA160 was introduced, however, I replaced my Pass X600.5 with it. I chose the XA160s over the X600.5s because—like many preceding Class A designs and tube designs—they offered a slight advantage in terms of nuance in low-to-midlevel passages. They improved the air, life, harmonic integrity, and low-mid level dynamics of the music. They also tilted the timbre slightly towards the upper bass and lower midrange—which helps compensate for the bright upper midrange and close-in perspective of far too many modern recordings. Plus my main reference speakers—the TAD-1s and Theil 7.2s—have very extended highs and more upper midrange energy than most reference-quality speakers.

These differences between the X600 and XA160 occurred, however, as much because of amplifier and speaker interactions as because of the inherent sonic character of the two amplifiers. Moreover, I gave something up in switching to the XA160s. As any reviewer can tell you, there is often only a marginal correlation between the technical measurement of an amplifier’s power and its real-world musical performance in a given system. The X600s, however, had much more apparent power than the XA160s with my relatively power-hungry TAD-1s and Theil 7.2s. There was a very clear loss of high-level dynamic capability and musical energy and life with full orchestral music and grand opera, and not just with sonic spectaculars.

These differences were not significant with more efficient, easier-to-drive, or less-capable speakers. The Polk LSi-15 is efficient enough in any actual system and listening room that amplifier power is less important. It cannot reproduce the same level of dynamics as the TAD-1 and Thiel 7.2. The Quad 989 is a very good speaker, but lifelike, high dynamic levels are also simply not its forte. With the Polks and Quads, the XA160 was clearly the better choice, and one that did not involve any meaningful sonic sacrifices.

At the same time, the XA160 did not do as well with a truly difficult load like the Spendor BC-1. The amp loses nearly half its rated power into four-ohm loads, and my reference speakers are nominally 4-ohm speakers. It did not have the X600’s amazing capability to control the speaker almost regardless of load. This became equally clear in terms of some aspects of the Thiel C7.2’s performance at more moderate listening levels, and in control over the bass in the TAD-1. The XA160 is not particularly speaker- or cable-sensitive. In fact, it is much less sensitive than many high-end solid-state amps and many vacuum tube amps. It is, however, more sensitive than the X600.