This is likely a ground loop causing current to flow through your subwoofer audio cable because of voltage differences between the subwoofer electrical ground and your equipment ground.
- Disconnect the cable to the subwoofer to see if the hum disappears. If you remove the cable and the hum goes away you have a ground loop problem. If the hum persists without the audio cable your subwoofer may be defective or the subwoofer volume may be too high.
- If you have a ground loop locate the source by disconnecting equipment with grounds connected to your receiver with either a video or audio cable until the hum goes away. This excludes optical audio connections. Common causes are cable TV or satellite TV connections to the cable box or TV, different ground potentials caused by equipment plugged into different outlets, different electrical phase on devices and poor equipment design.
- If that does not locate the source try replacing the cable running to the subwoofer. Sometimes a defective cable can cause this.
- If the source is the cable TV either correct the grounding error or use a Jenson isolator. You can also consider using an optical audio connection from the cable box to stop this, but if you are switching video through the receiver this is not likely to work.
- If the source is the satellite TV you must fix the grounding problem on your satellite system or use an optical audio connection and remove video switching from the receiver.
- If the source is differences in electrical grounds you must either have an electrician fix the electrical ground problem in your home or use products like these.
- If the source is the receiver or subwoofer itself it may be necessary to send it back for repair or replace it. Only resort to this after exhausting all other options.
More information on this topic can be found in this great article by Bill Whitlock.