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

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.

Thanks for that description, Petrushka. That makes a lot of sense. I think it would also depend in large part on the speakers and how they are voiced. Some are warmer and more relaxing and some are more dynamic and lively, etc. Also, on a listeners personal biases and types of music as you write.

I'd love to directly compare the XA160.5 to the X600.5 in my system. It's tough though with no local dealer and shipping costs for such heavy pieces.