Whether the collapse of a stereo signal is done inside the keyboard, or outside in a mixer, the result should be exactly the same. If there are phasing issues from doing so (it's VERY tricky to get a stereo recording of a source to sound identical to it's mono version).

That's why good mixing boards always have a 'MONO' button on them, so you can check this constantly... Piano is one of the most complex sound sources, difficult to mike well and remain phase accurate over it's entire range (the mike to source ratios change as you go from low to high). Just about everything changes in SOME way with stereo piano sample sets, but some ARE better than others...

One of the things I like to do with the P. Creek sound files is play them mono. Admittedly, it's a 192kbps MP3, so you get a bit of smear compared to a WAV, but it is still possible to hear the likely consequence of collapsing that big, gorgeous grand down to mono and putting it through a keyboard amp or mono PA.

You would be surprised at how few of them survive this well....