I am using the Pass Labs X250.8 monos. Love the sound. A few things I found using these amps that improve on their already great sound are: 1) Better power cords. Don’t skimp on power cords. They will be expensive but worth it. 2) Dedicated power lines. 3) I isolated the amps from the floor using springs. The amps sit on four 48 lb/in springs making the Fn= 3.6 Hz, if I recall correctly. Big improvement in sound. Detail increased by quite a bit. The overall sound improved. 4) Added a patch of constrained layer damping material on the inside of the top cover. Primarily I didn’t like the clangy sound when tapping the top cover and the damping material took care of that. What I didn’t expect was another improvement in the sound. I find the Pass Amps respond very well to better Power, Speaker and Interconnect cables. Buying cables is not as fun as buying a new amp or speakers but the right synergy can be very satisfying.
I have not heard any other amp in my current system so I cannot judge what would or could be better. I have heard my speakers in a store with the Dan A amps and they sound really good too; but that was before my tweaks.