@audiorusty that sounds likely. I do wonder however where the offending signal is coming from. As I mentioned I feel like the sound is a 60Hz “megatone”, so likely AC from somewhere. And yes lowering the filter would stop a 60Hz tone from being produced. And it starts with a quiet rumble and builds, it’s almost surely a feedback loop. But, I do wonder how the sub “feeds back” the tone back into the offending input signal, since the output is acoustic as you say. This totally makes sense when considering turntables picking up vibration from speakers and infinitely magnifying it, but I can’t figure out how a sub would do that to itself….
I do wonder if there might be an interaction between the sub and my main speaker driver, somehow generating a micro signal on the speaker cable, which is of course connected to the high level speakon cable at the amp terminal…. That micro signal could then be fed back into the sub in perpetuity, which would infinitely increase the signal level. Probably a far fetched theory, but interesting stuff. I suppose I could test it out by disconnecting the speaker cable from the main speaker and seeing if the problem still happens.