This morning I tried using another PSU (Drok 48V 10A) with only one amp module connected and the behavior is the same
It changes with frequency and input level (it acts like there is a tiny speaker in the amp module itself). Here's a video:
NOTE: in the video, there is only one module connected to the PSU, the output is connected to a dummy load (but it doesn't matter what it's connected to), the input is being fed a sine wave at a high level (not enough to light up the clip LED, though).
1) Using only one module hooked up to the PSU, if I unplug the CH2 header, there is no change in behavior of CH1
3) I'm only using one module currently
4) I tried a different PSU that was physically outside of the amp chassis and there is no change in behavior
Those are wires connecting the TRS sleeve with the XLR pin 1 and then also to the jack's chassis ground. I've tried with and without the connected to jack ground and there's no change.