We have passed the stage where each new model jumps significantly ahead of the others as they are all so good these days, consequently the law of diminishing returns starts to kick in. (Manufactures have to spend more and more on R & D just to get incremental upgrades)

The only way forward is to go software and have all the arranger & control features of the Tyros 3, Korg PA2x, and Audya etc, combined with the best sample sets and VSTs out there. (Not as extras, but included as standard) and then you would really see an improvement in sound quality.

Just think about having the following, coming as standard with the instrument.
http://www.soundsonline.com/EastWest-Quantum-Leap-Symphonic-Orchestra-PLAY-Editions-pr-EW-177.html
http://www.soundsonline.com/Quantum-Leap-Ministry-Of-Rock-License-pr-EW-173L.html
http://www.garritan.com/jazz.html
http://www.native-instruments.com/index.php?id=b4ii
http://www.native-instruments.com/index.php?id=kontakt3
http://www.musiclab.com/downloads/listen/
http://www.toontrack.com/ezdrummer.asp
http://www.ilio.com/synthogy/ivory/index.htm#demos

Regards

Bill