I had a Sunfire Signature sub with the (same?) problem. It was in my living room video system. The sub was very sensitive to any power surges in the room. I eventually ran the sub only on a heavy extension cord to another circuit in another room. I recently sold the sub to a friend who has it in his video on a dedicated circuit - no problems at all. I found the lower you have the level control set on the sub, the sooner it will go to stand by on the "auto on" mode. Then it will stay off until a bass demand triggers it, then the muted "thump".
Daniel
Daniel