I did another session with the PAS last night, it was my third time with the PAS.
I played a Yamaha stereo digital P90, my friend on P120 and vocals; and another guy on a Gibson 1980 Les Paul. We played jazz standards.

The vocals sounded very clear and the diepersion was nice, you can hear it clearly. The guitar sounded clear too. Voice and guitar are both mono instruments and produce sound mostly in the mid range. A single Bose PAS is mono and has good mid range clarity and dispersion. You can stand close to it and it doesn't hit your ears too hard because you are on axis such that only one of the little 24 speakers is aimed directly at your ear.

The two Yamaha digital piano keyboards we played (P120, P90) both use stereo samples and their stereo piano patches suffered in mono. The pianos sounded sounded thin, less open compared to stereo, boxy (like the old Kurzweil pianos), lacking in presence and sort of brittle. The magic of stereo, that open spatial sound, seems somehow compressed and sounds as if it suffers from phase cancellation when played in a mono system. The Yamaha "Rhodes EP" patches faired better, though somewhat narrow sounding. Rhodes EP Patches are mostly mid range tone, far less wide frequency range than an acoustic piano.

I also noticed a gap in the frequency range, something was missing in the upper low range. The area between the lower mid range and the upper low frequency range seemed lacking. Perhaps a different EQ setting could help that. I also didn't hear a lot of "sparkle" in the high frequency range. Again, perhaps a different EQ setting could help that too.

I think I will pass on the Bose PAS. I realize need to play in stereo to maintain the full tone and spatial sound of my Yamaha digital acoustic piano. By the way, there are no mono piano samples in the current Yamaha P series pianos. We tried every possible way of connecting to the PAS: going direct L/R Mono Sum Out, Right only, Left Only, both Right and Left Channels. Also with and without a Mackie mixer.
We could not get the Yamaha stereo digital pianos to sound good enough in mono. It's their nature that they need to be played in stereo.