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

Mwilliams,

pre amp considered is Pass Labs XP-20.

In my present system there is ATC SCA-2 and I might as well keep it - !?

French_fries, So you haven't actually heard these amps? Why comment then?

Unsound, If you mean by "dollar for dollar: yes" one should buy the Class A Pass Labs amps, then I would tend to agree, though I'm sure there are situations where someone would prefer the A/B models. They do sell many of the X.5 amps.

Peterayer, no that's not what I meant. I was answering; "Yes, dollar for dollar" to the question: "French_fries, Are you saying the XA.5 have less control at the frequency extremes than X.5 amps?"

I believe i have a pretty good handle on what pass amps can do, and i listen closely to those who have bought (and sold) different models, as well as the reviews of those same models. based on the mere fact that Pass Labs would offer two parallel amplifier lines where one would be (IMHO) way more musical than the other, indicates that the NON-XA amplifiers have some VERY important features that certain listeners are going to prefer.
a close friend has a pair of ALEPH-1.2's (as well as an aleph preamp) and i have heard them on many occasions. i absolutely love them. but they don't have much power below 4 ohms. that has never affected the sound any time i was over there, but some people will say that is unacceptable nonetheless, and needs "fixing".
Nelson still makes First-Watt amps as well. wimpy perhaps, but very short
circuit paths. he has good reasons for making those, too. beats the heck out of me...

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.