going Pass Labs, do I need "Class A " ?

So Unsound, I think I now understand you. Given roughly $15,000, the XA100.5 has less control at the upper and lower frequency extremes than does the X600.5? I was not aware of that.

French_fries, I agree there certainly is a reason for having the two different lines. Both sell well, so people must prefer one over the other. Besides the power rating in Class A, I have not read a description of how they really differ sonically. I had always assumed that an XA160.5 could drive any speaker as well as an X600.5 can, it just is biased more for Class A. Perhaps someone with more experience than me directly comparing these amps can clarify this. I've never had an X or X.5 series in my system.

Regarding the Aleph series versus the XA.5 series: I have owned a few models in each line. The most similarly rated amps that I have owned are the Aleph 2 and XA100.5, both rated class A for 100 watts and both mono blocks. I used each with the Eggleston Rosa speakers (87dB, 5-6 ohm). The XA100.5 was much better in my system. More effortless, lower distortion, quieter, more control over the lower frequencies, better soundstaging. No comparison really. I much prefer the XA.5 in my system. But as I say, I've never tried the X.5 series.

Petersayer, of course your speakers, room, and listening volume levels amongst other things would have to be taken into consideration as well. As I alluded to earlier, there are other things besides control at the frequency extremes to consider too.

I've never found the XA160.5 to run out of power driving any speaker. Current capability is often more important than wattage ratings and the XA160.5 can output as much current (36 amps) as the X600.5. That's enormous.

Once you're talking about the big Pass mono blocks, the usual tradeoff of class A vs efficiency becomes more moot. The big XA.5s can drive huge loads and the big X.5s have enough bias current where the class A advantage is in a diminishing return territory. Here, personal preferences should prevail as the two lines are voiced slightly differently.

In the smaller Pass amps the tradeoff is more acute because you really can run out of power with the small XA.5s! (depending on what your drive, your music and room)

Petrushka, That seems like a good summary, though I haven't heard an X.5 amp
in my system. How would you describe the slightly different voicings? Is the
difference fairly subtle or quite noticeable? Thanks.

The XA.5s are sweeter, darker, more liquid, palpable and mellow/relaxing. The X.5s are livelier, faster/snappier, subjectively more dynamic and punchy. In terms of bass the XA is warmer and more diffuse (but not untight or lack control) and the X is leaner and tighter.

I'm speaking purely in relative terms as all Pass amps are sweet, dynamic, etc compared to the competition.

I like the XA for jazz, classic rock, etc and the X for massive orchestral or choral works, not so much because of reasons of power (the big XAs have plenty) but because that's how they are most tonally suited.