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 

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