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

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. 

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