One thing I can tell you, the changing speakers will NOT make a big change, keep in mind that the PSR SX900 has a different Amplifier and processor, THAT and the speakers makes the difference.
It would not make any sense if there were more than one reason to go to the next model. to justify not just the price, but the new Features, and is not just that, many other things, more S.A sounds, much more Memory for Expansion packs, 4 outs, 2 USB ports, can load Audio Styles, more and Better FX, etc. over all, a better sounding keyboard.
My 2 Cents
Manuel