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

I had gotten interrupted in my post and due to the time limit could not edit or complete it.

I had failed to address the midrange directly. The ARC amp has midrange bloom where instead of a good solid rendition of colors with the Pass the whole spectrum is laid out in rich warm colors. This in part is why the Pass amp has so much slam… it is very fast and without such a well fleshed out midrange highlights the fast rise in bass and really slaps you, as if nothing else was there. Voices are rich and pure on the ARC.

Back to perspective. These are two great series of amps the Pass x series and ARC tube. The Pass XA series is definitely closer to the tube sound, but not all the way there. There are people attracted to easily hearing the drummers foot slapping the pedal or highlight the venue and mastering techniques, or need massive power and the solid state amps like the Pass x series are an outstanding choice. But for great rhythm and pace… one of the most important drivers of emotional connection and musicality tube amps / components tend to get this better.

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.