Jazzvox: I fought the same battle with my R8 before I sold it, and to this day fight it with the DR-770. I don't think there's any way to individually control the instruments (by which, I presume you mean panning, attack/decay of the drums sounds, pitch, etc. for each drum).
About the best I could do was to record a drum track into the sequencer, then split each note value into a separate track. This would at least let me use the MIDI volume controller to affect how loud each drum sound was.
If you find another way. please let me know!