I certainly can't say the S900 has the "best" piano sound...Tyros, especially Tyros2/3, and G70, have it beat, but it works perfect for me, even solo...I like using the P85 to control it for solo piano.
I play around with the filter to take some brightness off to soften it for jazz.
It's not actually meant to be a solo piano...it just has to work well within the arranger accompaniment, and it does that perfectly.
With my Jupiter 8 I have about the best analog sound I could want, and with a Tyros3 (or even S900), I've got excellent digital (realistic) sounds...the B3 on the Tyros3 is incredible so I wouldn't lust after a Hammond anymore.
I may look into an S70 XS in the fall.
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.