Like Brad I've been using Pass Labs X250's for a number of years now. Definitely seriously cleans, lots of power, and matches very well with the Nautilus 802. For a bit I was using a McIntosh MC252 which I also liked - but relative to the Pass Labs I found it to be a little slower on transients, and a little bit thicker in its lower mid reproduction - giving a bit "chestier" sound. As I was battling at the time a propensity to hollow out mixes too much by either cutting or not filling in low mids I found that the Pass Labs led me to a slightly better perception in terms of having my processing decisions being more correct. As always YMMV!!
Best regards,
Steve Berson