Starkeeper,
Hey I owned the PSR-540, and upgraded to the 550.. Personally I wasn't too thrilled with the piano sound of the 540, but with both the 540 and 550 you can combine voices and just by messing around with the DSP get a pretty nice sounding piano.. That's what I did with the 550 I combined 2 piano voices, adjusted the DSP and I think it sounds great now.. Yeah that's one downfall to many of the PSR models.... Many don't have internal batteries for back up.. You have buy regular batteries and keep them installed to keep your settings if you unplug the keyboard.. It's not really that bad either. If you don't use the batteries to power the board they last a REALLY LONG TIME for memory back up...

Squeak