Very interesting. IMHO it sounds like a chorus may be switched on, or the stock piano sound has chorusing in it which makes this sound like the "detune piano" setting on many synths. It also sounds like the same sample was used throughout much of the keyboard (this would be easier to confirm if the other C-scale walk-up files were tested on the WK8). Like many of the Yamahas, I also detect a short decay in the overall sound that doesn't sound natural. But I like the sound of the very final high notes of this piece, which is a weak area in many piano emulations.

My initial reaction is that more multisamples are used in the Yamaha piano emulations giving a somewhat truer and fuller acoustic piano sound than the WK8. Thanks for giving this a try... you might also make MP3's of the walk-up midi files and send those along with Moon River to Simon at SVPWorld for posting.