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

No experience with X150.5, but low level listening with XA60.5’s we’re quite good.

on a related note… I tried PrimaLuna Evo400 pre with the XA60.5’s and it just didn’t work well… poor synergy

Somewhat similar to @audiojan I have a Evo400 pre that I tried with a Pass 150.8 and didn't really like it. I would not use it for low level listening to say the least.  My typical volume maxes out at 80 db peaks and it sounded slow, unemotional and non-dynamic. Maybe it needed higher volume levels to bring out the best but I don't listen that way. Ayre amps have a very black background that I find works perfectly for lower levels.  It's hard to explain but it's different from anything I've heard. I use an older Ayre V-5xe with the Evo400 and am currently listening at about 53.3 db (just tested using white noise and not very accurate iPhone NIOSH app) and miss very little of the music. 

@ghdprentice Superlative set up man! 🤯
I appreciate all  the responses, to be clear this is basically just a matter of opportunity. I really love the D400MKII. If I had the room, I’d grab the Pass and keep the ARC. I highly doubt the Pass would disappoint me at any level but at the same time I don’t think it would be a jaw dropping upgrade, which is why I’m hesitant. My intentions have been to upgrade the preamp next…but this Pass came alone and grabbed my attention.  🤷🏼‍♂️

Just speculation as I've never tried it but the Pass with a good solid state pre or less tubey pre maybe could do it for you. A Florida Pass dealer I know suggests VTL pre with Pass amps. They can be had used at pretty good prices. He pairs those with Wilson speakers if that helps at all.

@chorus

i can characterize the difference between the Pass x350 and ARC Ref 160s. The difference is profound. Both have house sounds of their respective companies.

The most obvious difference is in slam and detail. The Pass amp has notably more slam and the details are more up front and highlighted. Before you have a real positive reaction to that… it is not a complement. The Audio Research has much better bass… bass doesn’t sound as it if of a narrow frequency… but a full rich wide band rendition of bass… sounding much more realistic like would be heard in and acoustic concert. It hits your chest as opposed to being a quick slap. The treble has a very different character as well, the details are all there if you listen, but they don’t unnaturally stick out.. the brass of cymbals more like brass and less undifferentiated treble. If you go to an acoustic concert… you hear the same thing. But then put on some loud electronic music like Yelo and the bass and detail within the bass and the layer out wide frequency response is incredible with the Audio Research tube amp. I had planned on switching my Pass x350 back and forth with the ARC… but never did… there was no reason. Later I found out the ARC amp I was auditioning was not broken in… it sounded better and better over time. Overall musicality was far far better with the ARC tube amp… it sucks you into the music quickly… the Pass sucks you in to listen to all the details and contrasts of sounds. The Pass exceeds at high contrast (using a visual comparison), the ARC a higher resolution low contrast.
They are both very quiet amps. The Pass XA series sound more like the ARC… but not nearly as far along the continuum as the ARC.