A good monitor solution for piano sound on stage is a big challenge. In my experience there're at least four things which will destroy your own sound: 1) room acoustics and reflections if you are in noisy place (when you are playing outside this is not that much problem). 2) Sound system which is giving back to you all the unwanted frequencies 3) (related to 2) bad sound engineers who doesn't know how piano is supposed to sound. Usually they put too much bass and quite often treble also. Of course sound is bad to audience as well but to you it's kind of "monster piano". 4) Your fellow musicians stage monitors which are giving bad piano sound to you. All this can cause phase problems etc. etc.
More often than not, these four things make it quite impossible to get a good sound for yourself. Then there's also often a time factor: 5) you don't have enough time to fix these things as good as possible (there's maybe 15 minutes "soundcheck" between two groups).
Of course I don't mean to say that there are no good monitor solutions for piano players. My main thing is that bad sound usually comes, not from your own monitors, but from other things. Some time ago I did test some of those high-quality near-field studio monitors on stage, but my conclusion was that even if they work perfect on studio situation, they hifi sound is quite useless on stage. What I personally need is a good and powerful middle (and upper too) range and you need to have EQ to balance it to this stage sound. Bass is not so important - you will hear these frequencies anyway from other devices. As been said, a good stereo perspective is very important for a acoustic piano player. (This is why I'm a little skeptical about those "stereo" keyboard amplifiers)
