According to most, the best piano available for the Roland JV/XP series, is the one featured on the SRJV80-09 Session Expansion. Provided you have an XP30 or JV1010 Sound Module, that piano should be included as the Session board comes preinstalled.
Contrary to popular belief however, I think that while the piano on the Session board is fantastic, the Piano expansion board has way more variety. If you're a keyboard enthusiast and need to have a variety of Piano sounds at your fingertips, then te Piano Board is a must have.
In order to grasp the sure quality of this board, you need to hear the demo included on the Roland SRJV Demo CD (not sure if this is still available). The main stereo piano included on the Piano board comes from the Roland RD-600 Digital Grand, which was popular in the mid 90s, and according to Sound On Sound magazine, is better than the SRX-01 Concert Piano board that's been produced for the XV series.
The Infamous Epu.