I use a pair of powered speakers on my PSR-S900 so the sound is in stereo.

I found playing the PSR's stereo piano (Live! Grand) through a single powered speaker took away from the sound.

Ian

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Common misconception...size and weight equal quality and performance. Don't be fooled.
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.