That sounds like some interference that can be improved with shielded cable if you ask me. If your assumptions are correct, using DAP as a standalone unit should also have this problem. I doubt putting DAP next to macbook will have the same issue. Let's see if using shielded cable helps or not. Well, it's not ground loop as you're so sure of so I recommend you to stop jumping your guns to conclusions that might be wrong. Or are you still on about ground loop stuff?