Jack,
What makes this thumping even weirder, is the reciever was powered off. Everything was powered off, but the sub.
Ajax is correct, and so is Mojo. Unplugging the video processor power with the sub on creates a low-frequency transient on the AC house wiring, which the EP600 reproduces.
I'd also suspect the filtering on those wall-wart power supplies, which typically are poorly filtered or shielded.
Of course we all get lazy about turning components off before we connect or disconnect any part of a system. But Jack's lesson learned many years ago is excellent. Turn everything off first.
Regards,
Alan
