How is the Pass Labs X150.5 at low volume?

Having the owned the 150.5 and currently 250.5. I would suggest going with the 250.5 it's far better at all volumes. 

I agree with above post.i had the pass in my room for about a year and moved to the 250.5....the 150.5 is  just ok

Pass Labs is a nice warm sounding amp
just in my opinion not a great match with Proacs the lower input impedance will load down your preamp try a smaller simpler design from ARC .Proac and ARC are a great match or keep your D400 and get a RefARC pre I wanted to keep out of this but there was too many guys just saying how great things are without understanding your true question of Proac/ARC best, JohnnyR 

@audioconnection , when you said “the lower input impedance will load down your preamp” did you mean amplifier’s low input impedance?
I’m in the market for a class A amp and considering the Pass stuff, hence the question.

Pass Labs amps are exceptional having owned a number of them for forty years. But I would not call the x series warm… they are not cold or analytical. I have nothing but praise for them. if fed a warm signal you will get warm output. The XA series are a bit warmer.