I recently purchased a Roland TR-626 off Craigslist and here's a cool trick you can do using its MIDI.
Each of the drum pads can be asigned a MIDI note/number and it will play them in any mode unlike a TR-707. Once you do that after reading over the pdf manual and learning the basic functions now you can use the 626 or 505 to play any drum module or MIDI synth. I'm using it to play Logic's EXS Software Sampler and creating custom drum kits using analog synths. One down side is it only triggers short notes on a synth but that's good for bleeps, arpeggio type melodies. The 626 has a shuffle/flam function but the TR-505 doesn't.