This is what I hate about most sample library pianos. Most seem to be prepared by developers who have software chops but are relative newbies at recording. It seems to me that they set up a stereo pair in front of a piano, and think that by filling up their harddrives with gigabytes of samples from every note that they can compensate for their basic lack of recording skill.
The problem with stereo pair recording is that they get very unfocused and phasey in the middle of the keyboard (which is where it matters most). They can sound fantastic on the high and low notes, but sadly lacking in mono compatibility. Not that the majority of users would ever notice.
There is also the basic limitation of sampled pianos that the mics have captured the ambiance of the room, so as soon as you play more than a single note, you get layers of room sound that are all out of phase.
The professional companies (Roland, Yamaha, Korg etc) have much better skills at recording great sounds, and often their products that only use a few megabytes of data can sound better than typical giga piano libraries. I imagine that they have done some really tricky stuff like recording with well placed mono mics in virtually anechoic chambers, and they often use resynthesis techniques and convolution techniques so they can create perfect piano sounds that are great in mono and better in stereo, and respond to midi control data better than just triggered samples.
See if you can borrow a decent hardware digital piano, or consider some of the software pianos that use modeling algorithms instead of just samples.
Or - see if you can use your samples in mono - they might sound better just taking the left or right channel. Maybe split up your midi notes and use either left or right depending on which sound better. Or - mix in the opposite channel at a lower level where the phase cancellation is less of a problem. If you get is sounding good in mono, you can then pan it or use a good reverb to stereoize it again.
Maybe use a synth piano for body, and mix in a stereo sample instrument just for the tops, where the phaseyness is less of a problem ...