This is an issue that has been debated and misunderstood over many many years.

In a nutshell......
If you listen to stereo piano samples through a single PA it will ALWAYS sound cheesy. This is because you are summing the stereo to mono and you get something called "phase cancellation" which causes frequencies to cancel or re-inforce each other and the carefully produced samples are destroyed. This only applies to stereo piano samples, most other instruments (maybe in fact all) are mono so they are unaffected by playing through a single PA.

Your experience with a single Bose PA is exactly what happened to me when I bought a single Bose Compact PA a few years ago. I was terribly disappointed by the cheesy sound from my Yamaha stereo pianos - other sounds were fine. To cut a long story short I bought another Bose Compact kept the L/R outputs separate and the beautiful Yamaha piano sound was back. It also helps if you pan the L fully left and the R fully right.

You will get lots of other suggestions, including using your piano mono outputs but at the end of the day, stereo piano samples are meant to be played in stereo and there is really no other way round the problem. That's why your piano sounds so good using phones.

Tony