I had a similar problem years ago and it ended up being the subs auto turn off was not functioning. What you could try, if you must leave your receiver on, turn off the speakers switch if you have one, mute the whole thing or put your receiver in standby. Try these and see after 5-10 min. if the sub turns off. If the sub still stays on, disconnect the sub speaker input and wait again. If the sub stays on, which it should not, then the auto on/off is shot. Sub amps are not meant to be on and idle all time. As a last resort, if you want to cut power to your sub without turning it off, if you can, plug it in the receivers switch recepticle and turn your receiver off or standby.
If it is a grond loop and your sub has a 3prong cord, try a cheater plug.