Hi Stu!
It sounds like you have a ground loop going on. There are a few real quick troubleshooting steps you can take.
1) Remove the ground screw on the amplifier at the rear of the sub. Does the hum go away?
2) Try a 3-2 prong adapter on the end of the subwoofers electrical cord. Does the hum go away?
3) If the answer to 1) is yes then disregard step 2) and your problem is solved. If step 1) does not solve the hum proceed to step 2). Does the hum go away?
4) If step 2 solves the hum then we have to proceed with some more troubleshooting on your setup to remove the ground loop without using the cheater plug. Ground loops are a common issue and can be solved with some patience and being methodical. It's not a flaw with the equipment.
