I find the Yamaha Stereo Grands just a tad 'phasey' even on normal speakers. It just, IMO, gets a little funny in the middle, the edges are fine, but the middle of the keyboard feels just a bit hollow.

How well does it collapse down to mono (say you just use the mono out jack instead of running stereo)? Does it sound weird compared to when it's in stereo...? That's usually a sign of poor phase alignment during the sampling sessions, and is very hard to get rid of if it's in the samples themselves.