At least you've eliminated conducted or radiated interference on your sub cable. So it appears to be coming from the power outlet.

What other equipment is plugged into that same circuit that is powering the sub? Try unplugging (not just turning off) all equipment that is on that same circuit. Then see if it stops. If it does, methodically turn each piece of equipment on and monitor to see if the thump comes back. This way you may be able to isolate it to a specific appliance.