@dman777
Not sure what your definition of liquid sound is. Is it warmth or is it the euphonic sweet, silky type of sound. If it’s the latter I don’t know if either Pass or Moon fit that description.
I’ll give it a shot so you can decide if it’s helpful or not…
I’ve owned X250.5 driven by ARC tube preamps, XA30.8/XP-22 and X260.8 mono amps with XP-22 and test drove ARC Ref6 with the X260.8 for a couple of weeks.
The Class A XA is the warmest and the mellowest sounding with somewhat wooly bass and paints the soundstage with a really big brush. Very pleasant sound if that’s what you’re after.
The X260.8 monos are much tighter, have more control and more bite with better instrument separation and better details. Still warm sounding and has the typical Pass Labs homogeneous sound signature that makes everything sound good. It won’t render a lot of recordings unlistenable. It will not be analytical sounding and won’t be very exciting sounding either. The X350.8 will most likely adhere to the same general sonic principles.
The XP-22 preamp is excellent. I couldn’t really decide between the ARC Ref6 and XP-22 as both were extremely capable. I stayed with XP-22 as to me it portraits the recording more accurately than the larger than life presentation of Ref 6 which is amazing in its own way. I’d be happy with either but I already owned the 22 and decided to stick with it at the time.
I no longer have the Pass stack as I simplified my system quiet a bit. I’m running a Boulder 866 integrated now. There’s not been a day when I caught myself thinking man it would be nice to have that Pass stack now. No…I kind grew tired of the homogenous presentation after years of owning that gear
Have not heard Moon North so can’t really speak to it.
As @audiopacer stated, Pass amps run hot. Not an issue in a large air conditioned room but you will feel it.