My Yamaha P120 and my Yamaha P90 digital pianos have only stereo piano samples, no mono piano samples. When playing through a mono sound systm, my Yamaha stereo piano sounds, and other brands too, suffer and become thin, edgy and almost all attack. This is the case for most stereo sample keyboard sounds. When the left and right samples are summed on top of each other into mono their is an out of alignment problem that causes a chorusing and phase cancelation effect.