I had the XA-25 and the X150.8 in my home at the same time for a demo. Reno hi-fi, which is gone now, but was an authorized Pass Labs dealer, graciously loaned both to me. I listened to them primarily on Martin Logan Impression 11s, and also on a pair of KEF ls50s. The XA-25 was very clean, "water clear" is the term that kept coming to my mind. The X150.8 was bigger, more fleshed out, maybe a little warmer, and more engaging on the electrostatics. Maybe because the Martin Logan's can be a little dry and overly analytical. In any case, for me it wasn't even a contest, The humble X 150.8 just sounded better and more musical. I brought the XA-25 back to Reno hi-fi and kept the X150.8. I still consider it to be the best sounding amp I ever had on the Martin Logans. And I had a lot of amps on those speakers. The folks at Reno hi fi told me they weren't surprised, and that the XA-25 was so well reviewed that a lot of customers were disappointed and several ultimately traded it in for something from the 8 series. That of course is just one man's experience (mine).
Cheers,
Ron