I have the exact same setup. I really wanted to route the A4 into the MD, so I could do RAM reverses etc… but it dulled the sound too much. When the MD is routed into the A4, you do not waste any tracks, and the sound quality is awesome. Actually, I find the reverb on the A4 is wicked, and just a small amount on the machinedrum signal sort of warms it up and rounds it a little.
Cheers:)