Yamaha's promo on the PSR9000 says "Yamaha's brand new Live! Voices-acoustic instrument and human voices such as Nylon Guitar, Strings, Gospel Choir and Scat Vocals-feature stereo samples to ensure the most accurate reproduction of the instruments' natural presence, resonance and vibrato...". They are playing up the stereo sampling of these voices so unless I'm missing something, more than just the piano is in stereo.