Cassp,
I have several clients using the NP30 with the S900 and/or PSR-3000...they are quite happy with the combination.
Personally, I don't like semi-weighted actions, preferring either a really light touch or a weighted hammer action.
A proper stand for the above setup is essential.
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.