If it was my money (but, I know...it ain't), I'd go with the PSR-S910...well worth the bit more, and, as Gary has said above, no reliability issues whatsoever.
Plus, it uses the new style engine as in the Tyros3/4.
Personally, I found the S910 quite a substantial improvement sonically over the S900...the sound just had more punch and yet was sweeter as well.
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.