Pull the Y cable out and plug your 'line out' cable directly into each side of your power amplifier input individually to isolate.
I've tried that. No change. But I'll make sure it's set up that way for future tests.
Try it without the charger in the circuit.
Charger isn't in the circuit except when charging.
Try shielding the audio inputs.
How? Use shielded RCA cable? I've taken apart two RCA cables and only one had any kind of a braided shield, and that also functioned as the ground connection. I'm aware I'm supposed to only ground the shield on one side, but I don't see how I can accomplish that with a standard RCA cable.
Work backward from the speakers to inputs(audio and power).
What should I be looking for?
These all seem like good suggestions for isolating noise from the circuit, but even if I succeed in removing the noise by adding shielding, seeing as the Mighty and Amp work fine together when they don't share the same power source, correcting the issue with more shielding somehow doesn't really seem like it's getting down to the root of the problem, which the evidence seems to be indicating is the result of a ground loop. But I don't have a clue how to break said ground loop.
I found this document by TI with circuits for breaking ground loops:
...and it seems to indicate that a 5 ohm resistor on the ground of the RCA cable should do the job. And I found some suggestions elsewhere that a 10-100 ohm resistor there would work. I didn't have any 5 ohm resistors, so I tried the 100 ohm which I assume should work but might act as a filter for some of the signal I want to keep, but that didn't make a dent in the noise.
Now that I look at the "Ground Loop Noise Reduction at an Output" section again though, that doesn't seem to match the schematic I got from the other site. I put Rgbk where Zc is. It appears though that Rgbk is actually connected between the ground of the device and the ground of the RCA output. So if I connect a 5-20 ohm resistor between the Mighty ground and the ground pin on my RCA output...
But looking at my own schematic again, the ground pin on the line out goes directly to device ground. And their schematic seems to show an op amp whereas my output is from a dac. So I don't know what I should actually be doing here.