Hi!
If it's piano samples you're after then having to choose between several GM modules seriously limits your options.
Some may have piano samples that are a tat better than what you've got in your SC33 but don't expect a radical difference since the total sample ROM on most GM modules is only a few megs. An entire library of sounds squeezed into a few megs means short samples and relatively low quality. can't have that if you want a good piano.

what you could look into instead is perhaps a synthesizer workstation with good on-board ROM and GM compatibility (Press GM and you're in a familiar world. Except the samples are longer and the sounds are better in quality).

I'm not saying you should run out and buy a Motif ES or Phantom and use them in GM mode. But getting out of "GM only box" will surely give you more to choose from. Again, if it's just a few basic sounds you're after then most workstations and hi-end modules have a GM mode.

yamaha XG standard is another thing to explore. More variations of the same sounds. cleaner FX and more flexibility with editing (nothing that's rocket science though).

Another alternative is software. There are some good software instruments and GM sound fonts that feature much much much better piano (and other acoustic) samples than any hardware. Of coarse if you're not setup to use software instruments this isn't something that you can explore at this point. Consider it though.

I mean next thing you know you wil be playing Spectrasonics "Ivory" along with some nice giga-GM set and laugh at all the GM stuff that you were considering.

Sorry I can't answer your question about that yamaha but generally I find Yamaha synths a tat warmer sounding than say Roland synths. Not having heard the module you're talking about I can't say if it's better or worse than your SC33. I've used SC33 though and I can say for sure that it's pianos suck. They were good when the SC33 first came out but times have changed. So no wonder you're considering geting rid of it. It's old junk man. Used and loved by many of us but it's history nevertheless.

So here you go. That was my $00.02 for you.

-ED-

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A gentleman is one who never hurts anyone's feelings unintentionally.
- - - Oscar Wilde