I live in rural TN, so I need to drive quite a ways to find a good audio store, Atlanta or Chicago most likely. Most of my real listening is at headphone meets that I host. This is how I had the M21 at my place.
I have a huge room, about 15’x30′, 14′ ceiling and open to kitchen and dining room. I had Quad 2905 in there before and moved them to my office when I acquired the soundlabs. Quads now have bass in the office.
When I first acquired the SoundLabs, I drove it wish Pass XA30.5, mostly due to that is what I had. Bass was lacking, not really controlled, so I was wanting more power.
TWB drives his soundlabs with xa60.5 and the smaller soundlabs with x260.8. http://www.soundlabowners.com/forums/topic/atma-sphere-ma1-3-2-vs-pass-xa60-8-amp/#post-402
I would also like to hear the x350.8, but my preference to class A and TWB’s opinion that 60.8 was powerful enough and reading lots of reviews, I planned on getting XA100.8. I found a used pair of XA160.8 for a good price and was happy to have more power to the SoundLabs. The XA160.8 definitely controls the bass. I kind of dream of XS300 or XS150, but they are very rarely on the used market and not inexpensive. I don’t think the XA200.8 would sound much different. I think I have plenty of power now. I usually listen at a relatively low volume, though I can turn it up quite loud and it still sounds exceptional. I also have the SoundLabs 922 (22 degree dispersion), thus my speakers are effectively more efficient due to not dispersing 45 degrees.
I remember reading a review of your CATs with Soundlabs, I found a pair that I was quite tempted with, though I think they were 240V and I would have needed to add new circuits to my listening room to accommodate them.