My P-5 also sounds a bit dry, but not egregiously so. I agree that some cable swapping is in order. My suggestion would be to try either Wireworld Gold Eclipse 5 or Acoustic Zen Reference Matrix II in XLR configuration out to the amp. Borrow cables if you can, from dealers or friends, to sii if they might change the sound in directions you like.
The suggestion of trying a used passive in place of the P-5 is also a good one. It might be less expensive than buying new cables. I bought a transformer based passive, and it sounded noticebly warmer and more musical than the P-5. But given that the P-5 is such a capable preamp, it's really worth trying to bring it on song first.