Hi Mikeyb,
Ian means that you should connect the speaker cables from your receiver or amp to the high-level inputs on the sub and make sure the cables are connected to your receiver or amp as well. Don't connect the RCA low-level input at the same time as the high-level inputs. Turn on your amp or receiver and see if the hum goes away.
You either have an AC ground loop, likely caused by your cable-TV or satellite connection (if you have either) or traceable to other components connected to your receiver, or there's a defect in the amp in the subwoofer.
Try the remedy above first, then disconnect your cable-TV connection from your TV, if that is part of your A/V system. See if the hum goes away. Let me know the results of these tests first, then we'll take it from there.
Regards,
