OK, let me restate my conclusion without any mention of phase cancellation.

When the stereo is summed to mono inside the keyboard the quality suffers far more compared with stereo than when you use a mono piano patch.

I might also think there was an issue with my particular keyboard except that there has been a huge amount of testing done with different keyboards going back to 2004. It is documented on the Bose forum and although some people find the summed sound acceptable, there is no doubt about the loss of quality compared with a mono sample.

Yes Mac, I enjoyed my first gig and have another on Xmas day. I and my audience love my mono piano!

Merry Xmas
Tony