PSB Synchrony One Speakers and Pass XA30.5 Amp

Why change the amp? I suggest auditioning some speakers that are more efficient and are more neutral. PSB's are toward the warm side, which is not a good match for the Pass Class A.

I have heard those PSBs paired with Bryston monoblocks and they sounded quite dynamic and very musical. Aside from your room configuration - on that later,  your issue as said before could be a matching issue. Your space arrangement seems to suggest you have a ~14' open space behind your listening position which could be a problem. Your 16' ceiling is another potential culprit. Also, I had a very similar experience but with completely different components where I had a real long and narrow room with tall ceiling and I could never get the near-field set up work for me using basically half of the length of the room. I moved everything to a smaller (repurposed) bedroom - 8' ceiling but the same exact distances for near-field listening and it all came alive. Try some temporary room treatment, especially behind your listening position and see if it makes a difference. Finally, looking at the measurements for these PSBs, a large portion of frequency band operates in 4 or less ohms - particularly the area where most of the music energy lives so by all accounts these speakers are putting a high demand on your amplifier's power supply.

Why change the amp? I suggest auditioning some speakers that are more efficient and are more neutral. PSB's are toward the warm side, which is not a good match for the Pass Class A.

I'm willing to consider alternatives to the Sychrony One. What can be recommended in that price range, say, up to about $4k used? Has to be full range and good looking. And good with all kinds of music, but jazz, vocal and classical, especially. Ideally one known to match well with the Pass XA30.5.

Look for a more efficient speaker (over 90dB sensitivity), preferably 8 ohms, which rules out Dynaudio, Totem, B&W.
Kef, Klipsch, GoldenEar are efficient speakers that can be driven by tube amps and are more transparent than PSB. Even if you were to move to a more efficient PSB model, I don't think their warm sonics would match well with the Pass.

Perhaps, start a new thread and you will get responses from tube guys and Pass Class A owners who can recommend an efficient speaker that excels in jazz and classical reproduction.

Just like any other "which speaker" question, you'll get dozens of different responses. Make it more difficult, there's a whole another divide between classical and jazz content which tend to be polar opposites as far as speakers go. And to top this of, ~$4K is another  sweet spot for speaker options - just like $2K, and sky is limit in terms of options. Double your search if you are open to used. Enjoy the journey.