Try a factory reset on the S900...hold the uppermost key(C) whilst switching on the keyboard.

Someone may have been messing with it.

I get no distortion on my S900 when raising the gain.

Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.