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

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.

Hasmarto,
yes the pass amp will load down your tube pre amp
like I said .
what part of my statement  don’t you understand.?
try to listen to your ARC amp. With an ARC Ref level Pre amp
and you should be in great shape
JohnyR

@audioconnection , I don’t have a ARC amp, and never will, I’m looking for class A Pass amp, and the input impedance is 50/100 Kohms, SE/Balanced, so kinda thought you’re implying that their (Pass) input impedance is on the lower side, wanted to clarify the issue.

No need to be defensive here, I’m not your enemy, just read carefully, and I’m not the OP.

Have a good day.