I have set up the Bose AND two powered speakers on three occasions, side by side, and a-b'd them. In my opinion, and in the opinions of those that were there for the demo, the Bose sounded better, whether using styles or solo piano.
Right now I'm using mostly the Roland E50 keyboard. I'm not thrilled with the pianos, but it has nothing to do with the Bose.
I've tried hard to hear any phase cancellation in the pianos, and experimented with Tyros 2 and S900. If it's there I can't hear it.
Regarding stereo; I feel the Bose is superior because you can better distinguish all the nuances and sounds that are sometimes lost with conventional systems, particularly when those systems are not properly equalized.
Hope this helps.
DonM