Quote:

If you use only the L output on the piano, with nothing at all connected to the R output, then any Patch you pick should be summed to mono. It is that way on every keyboard I know that has audio line outputs, regardless of brand.



Exactly, and because the stereo is summed to mono inside the keyboard the quality suffers due to phase cancellation between the left and right. The only way to preserve quality through a single (mono) amp is to start with a mono piano sample that has never been through a summing process.

Tony