@ddafoe Which Pass amp do you have and how does it compare to the GaN amp?
I owned the X250.8 for maybe 5 years and switched to the XA30.8 a couple years ago.
I find Pass to be very smooth, a tad warm, offers zero fatigue, very natural tone, and throws a killer front to back soundstage. So far I find the Atmasphere GaN monos to have a very similar sound to my XA30.8.
"very smooth, a tad warm, offers zero fatigue and very natural tone", check.
"throws a killer front to back soundstage, ...", is slightly different but just as engaging I think so far at least.
I find the monos offer a maybe slightly less 'meat-on-the bones', but a wider soundstage, good depth but maybe not quite as deep, but offer a blacker background and ultimately allow me to hear in and around the instruments just as much, just in a slightly different way.
When I get more time I plan to swap back to my Pass and compare the higher frequencies. That is one area where I haven't made a decision yet on how they compare. I find the Pass amps to offer very nice high frequency detail and refinement, without being bright or forward.
I can see why tube amp owners or Pass owners would like these. Personally, judging them based solely on their looks, power rating, or because they are GaN based as some folks have done doesn't really hit the mark in my view. They definitely sound different and very Pass like compared to most amps I've owned. Some folks may love them and some not so much, but in my opinion they are not the type of amp you just throw in as just another Class D or GaN offering. They are definitely worth a listen if the sound characteristics I mentioned above float your boat...