In fact the X5 series can be tweaked to produce a much improved piano tone. If you are willing to use a PC or Mac to program it, with the Korg Editor program, you can find excellent patches on the Korgy Park site. I have a 'Stereo Piano' programmed from the 'Piano 16' sound which is waaaaay better than the yucky presets.

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Etcetera prog rock: http://users.cybercity.dk/~dko4342/