The original Roland units had per-instrument outputs along with some trigger outs. The TR-8S doesn't offer these, instead going with a stem-like approach with three assignable output pairs. While this might seem convenient, the fact is that having the individual outputs offers MORE leeway in sound design, which both the RD-8 and 9 have. With that, it's possible to separately process each individual output if desired.
-- Lugia
I noticed that when looking at the RD-9 page that it has an out for each drum sound...I need to read up on the 'stem like' approach of the TR-8S...may be good enough for my amateur ass at this point ;)
JB