I'm perplexed about this buzzing problem.
Did you use the same power supply with both DL4's?
It sounds like you've tried just about everything else. The only other thing that I can think of, is that maybe you've got a feedback loop going somehow - that will cause a high-pitched tone, although it doesn't seem a likely culprit. If you have a barrel connector, try connecting the send straight back to the return, and see what happens.
Regarding overloading your EDP: I used to have that problem a lot, especially with low-frequency signals. Then I realized that the EDP simply doesn't have as much headroom as the Mackie, so I did two things: (1). I turned all of my levels down, so that I was no longer peaking out at +10dB, but rather at unity if possible. (2) I turned the EDP's input down a little and then boosted its return channel into the Mackie a little bit to compensate. This won't do wonders for your signal-to-noise ratio, but neither will clipping.
When the red light goes on, things do tend to get ugly. You can hit UNDO as soon as you get distortion to keep the crunch from looping, and then back off the feedback a little bit to let the levels settle down to a manageable level. With your tuba, I'm sure that you must get big pileups of waveforms, which are maxing out the looped signal. I believe that the EDP software automatically cuts the looped signal somewhat whenever you overdub, but it may not be enough when we're talking about Big Bass Waves. You may need to keep the feedback rolled off a hare to keep things under control when you're overdubbing a lot of low notes. If you're using a MIDI pedal to control the EDP, you could theoretically assign two pedals to the overdub function: one regular for higher notes, and a second one that lowers the feedback while it's pressed down for the brown-tone drones.