This response does not directly address the difference between midi and real track piano quality but it may be relevent in understanding what is happening.

I was always very happy with the quality of the stereo piano sound from my Yamaha keyboards when played through a stereo amplifier to two speakers. When I started to use a single Bose PA I was terribly disappointed with the piano sound, it was "cheesy" and lacked the clarity of the great Yamaha piano sound. All other instruments, both synth and RT sounded great.

I did a lot of research, particularly on the Bose owners forum, and eventually found the reason. The stereo piano sound relies on keeping the left and right channels separate using stereo speakers because the source is generated from left and right positions across the width of the piano. All other instruments are "point source" in comparison. If you combine the channels and use a mono amp (like the Bose) then you get, what some people call, "phase cancellation" and there is a massive deterioration in the quality. Summing stereo to mono with piano sounds just doesn't work well. All other instruments sound no different using a single Bose - just the piano.

After exhaustive testing I eventually bit the bullet and bought a second Bose for performing and the Yamaha stereo piano sound is back. You have to keep the channels separate right to the speakers so I use separate mono inputs in my mixer with one panned fully left and one fully right.

I haven't experimented with the difference between midi and RT piano sound but because you are using a single mono Bose I suspect the reason is something to do with the difference between the way the sounds react differently when you sum to mono via the Bose. I have been suprised how this phase cancellation issue with stereo piano sounds isn't more recognised.

Tony