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From Stereophile's review of the XA25, we learnt that like the XA-30.8's published 30/60wpc spec which scales very comfortably to a nearly triple 90/150wpc into 8/4Ω when THD is relaxed to 1%, the smaller amp's 25/50wpc rating then reads 80/130wpc. Or as John Atkinson commented on those measurements, "... it appears from the shape of the traces in these graphs that Pass specifies the XA25's power as when the THD+N is close to 0.01%." To paint the connection between available power and distortion with an example from that other part of town, consider the German Audio Valve Baldur 300. It's a tube mono with ten 6AS7G power triodes per channel. Its peak power is a massive 300 watts. At 1%THD, it's still a mighty 240. At 0.3%THD, we're down to 100 watts. If we wanted half that distortion, we'd end up with just 10 watts. Yet 0.15%THD would still be a decimal point higher than the 0.01% which JA measured for the Pass. And that makes 25 not 10 watts, does so with less noise and lower output impedance and sells for $4'900 not £22'000. Strong talking points for transistor lovers.


In his review, Terry London reminded us that, versus the XA-30.8, the XA25 increases damping factor from 150 to 500, lowers output noise from 200µv to 50µv and doubles slew rate from 50v/µs to 100. Finally there's Stereophile's measurement of a 10kHz square wave with very short rise time and no overshoot or ringing. Clearly the XA25 has some impressive figures going for it. In the same or at least next breath, we admit that none of it tells us what it sounds like. For that we need our ears. Accompanying hardware for my first auditions had our usual suspects: iMac FusionDrive digital transport or Soundaware D-100 Pro SD card reader; Aqua Hifi discrete R2R DAC; Wyred4Sound STP-SE II preamp; Audio Physic Codex speakers; and for first contrast, the XA-30.8.


Two stack shots show the XA25 to be narrower, lower and shallower than the 30.8, i.e. more compact in all three dimensions. What it doesn't show is how much less it torques your lower back. Age becomes inversely proportionate to joy over raw component mass. If something must be big and heavy to work—class A bias at significant power pretty much dictates it—one accepts the fact that reality bites. But size for the sake of size or to impress anyone with? That certainly dissipates as a motivator. On that lumbar score, the XA25 had the obvious advantage. Having run the XA-30.8 over its XLR inputs for a while, I now had to revert to RCA to keep comparisons simple. On the matter of balanced inputs, the XA-30.8 has the edge.


For thermal readiness, both amps had 40 minutes of idle roast'n'toast before I cracked into any tunes. First up was Mayte Martin's Al Cantar a Manuel. It marries high production values of guitar, violin, bass, drums and cajon to vocals which seem to expose the uvula, that wiggly little worm inside Mayte's throat. Here the XA25 managed an unexpected troika of attributes. Tonally especially in the vocal band it was slightly fruitier than the XA-30.8. Microdynamically it was clearly more expressive like a faster circuit. This was particularly evident on Mayte's voice. It scaled more dramatically, hence felt bigger. When bass and drums kick in to suddenly light up the soundstage in its outer quadrants, their contrast ratio relative to the guitar, violin and singer which were already on was higher. With the XA-30.8 in my estimation being slightly dark, bassy and dense, the XA25 stirred in some quicksilver. The mean trick was injecting a quickening agent without diluting the tone density and chunkiness I adore in our stable mate. Typically, so-called fast amps like our 1MHz LinnenberG Allegros lighten the load. It's as though to go so fast, they have to shed some tone mass or physicality. Decisively, the XA25's uptick on the accelerator did not. The core beneficiary I made out on this first track was higher dynamic contrast within the range of a first-rate singer, i.e. not the intense climaxing of symphonic peaks nor the envelope of a heroic tenor at his best but the emphatic expressiveness within a non-operatic melodic arc. Audiophilia calls that microdynamics. As the XA25 showed, those needn't be so micro.


For my second pass, I transitioned to virtuoso Arabian chamber Jazz compliments of The Khoury Project's Revelation, a prior year's best find. It showcases violin, oud and qanun against bass, hand drums and percussion. Again the XA25 took the lead in dynamic range. It also notched up the frisson or electrifying aspect of drum hits, cymbal slaps and wood stick cracks though neither amp peeled those out from the surrounding thicket like our ultra-bandwidth LinnenberGs do. Ditto for harmonic envelope. The Middle-Eastern violin played as modal fiddle and the region's lap zither can scratch and splash, respectively. It takes a leaner amp with more treble exposure to maximize those coarser rawer aspects. By implication, such amps will also reduce some tonal mass and colour saturation in trade. Yet over the XA-30.8, the XA25 did reach a bit deeper into the bubbling cauldron of fire flies and metallic glitter as those scintillating refractions which will come off strings played hot. On these Khoury tracks, I also appreciated how the previously mentioned enhanced contrast ratio tied directly to the XA25's slightly quicker percussive transients and somewhat more open overtone window. Finally and admittedly unexpected, the XA25's bass was a bit more emphatic and potent. These small but significant gains didn't come about at the apparent expense of anything else. At least on these speakers, the smaller amp began to make a most reasonable appeal to downsize yet get ahead. Time for the king of instruments, some piano.


For a double billing of Classical and Jazz plus high production values, this meant Telarc with the Jacques Loussier Trio plus string orchestra doing Mozart's Piano Concertos #20/23. For some reason, now I first tuned into how fundamentally similar these amps were in their sweetness. Coming from the Bakoon, Crayon and Goldmund/Job aesthetic as my benchmarks for the 'Swiss' sound school, that could nearly be called minor lushness. It's an aspect where tube notions might arise—justifiably so—should one's idea of solid-state amps be synonymous with the leaner more agitated aesthetic. That tube amps of my acquaintance generally weren't as linear, quiet and in control is another matter. But it explains why, rather than to actual valve amps, I've transitioned to Pass Labs and FirstWatt. Their amps do some of the things I like about tubes without any of the things I really don't. By now quite certain where to look, the higher orchestral background violins here and there showed just a touch more horse hairs with the XA25 as their metallic/glassy not wooden/creamy personality trait. This led to the impression of elevated image lock or being better separated out from the mix as discrete entities. Still, the overriding takeaway from this session was somehow more focused on how the amps overlapped, not where they diverged. The key word of complete agreement between XA-30.8 and XA25 was sonorous.


If we consult certain synonyms, we circle that sonorous wagon more tightly. "Vibrant, full-bodied, resounding, majestic, lofty, strong, imposing." In audiophilia, the most popular catch-all for that entire bag is probably bloomy. As a quality it sits directly across from overdamped, dry and lean. Considering how well these amps controlled our hidden woofers, it could seem surprising that effective LF damping didn't simultaneously dry out the remaining bandwidth. It patently did not to probably point not so much at low output impedance per se but how Pass achieve it. So this more redolent general behaviour could be another quality which, coming now especially from the class D school of massive negative feedback à la nCore, could strike many as more tube than solid-state. Obviously such an opinion expresses a narrower exposure to the many flavours transistor sound can offer. Still, in its limited fashion, it makes a very valid point. Neither of these transistor amps does is dry or metronomic. But as Herb Reichert's Stereophile comments made clear vis-à-vis an 845 Line Magnetic valve amp, tubes still do it differently. Now rather than tick off more distinctive points against the XA-30.8, we'll save some for the FirstWatt F7.