As always, pianos are subjective, but that EXB4 piano is a dog..... dull, no sustain, weird jumps in timbre, the only thing that could make it acceptable is comparison to the abysmal on-board piano!

Have you considered an outboard rack or module? A used Roland XV2020 or similar would allow you to access their SRX-11 Complete Piano soundboard, probably one of the best hardware pianos out there.

Alternatively, how about looking at the sampled pianos for Akai (some may have already been converted for Korg) and use the 32MB of sample RAM you have.

While the PA800 and Oasys have shown that finally Korg are starting to understand how to sample a piano better, these older efforts (EXB-4) show how far they needed to go.

Don't forget about still listening to ALL available sampled pianos at http://www.purgatorycreek.com/index.html to get a direct, apples to apples comparison. Eye-opening.....