Well the AKX10 isn't meant to compete with an sx900 (not even an sx600, in fact). It *does* appear to be in the ballpark of the PSRs650 or s670, but with a very slick PSRSX inspired touch screen. It appears to be really good for the money, and if your Yamaha was a PSRs900, PSRs710, or PSRs650 or s670, I would say the AKX would actually be an improvement of sorts; but you would lose out on that huge, huge Yamaha style database, especially since the sx900 is at least Revo compatible, which none of the other PSR's (or Medeli's) are.
Mark