I don't think I've ever done a sequence on either my JV1000 or my XP60 in which I didn't double some and sometimes all the drum parts -- "all" is easier on the JV1000 because you have the separate GM kit on the Voice expansion board, and often a GM sound is all you need to bring out the character and add punch to the more exotic sound on, say, the Hip Hop board etc. For the XP60, I'll use sounds from one of the drum menus on one of the expansion boards or use a user or preset kit to enhance the sounds on the expansion board (even in beatloops).
I try to determine which drum sounds are not what I want (not enough punch, not loud enough, not cutting through the mix enough, not enough snap, not enough sustain on a cymbal, etc.) and then pick a sound in another kit or menu (or even in the same kit) and double the original sound 1 or 2 ticks/clocks later than the sound I'm trying to enhance. Cymbals can *really* be enhanced this way, as can especially snares and bass drum sounds.
Generally not *all* the sounds need enhancement or help (closed hi-hat, rim shots, some shakers etc.), so you don't really end up using up that much more polyphony, and most of the drum sounds are only one voice anyway.
Occasionally a Juno-type bass waveform can soup up a bass drum sound, or a rim shot can give a snare extra snap. Especially if these extra sounds are on another channel, you can tweak their volume as well so that it's more transparent.
Finally, remember that you can program your own 4-part drum/percussion kit as a normal patch and save it to a user memory slot.
doug