This could be a ground loop hum/buzz or possibly a transformer hum through the speakers. In fact, large subwoofer transformers are more susceptible to a slight hum noise.
A speaker hum at your listening location is never normal. To determine if the sub amp is functioning correctly, the best thing to do is disconnect all of the connection inputs going into the subwoofer. Keeping the subwoofer plugged into the AC outlet, turn it on; if the hum still exists, there may be a ground loop issue or a problem in the sub amplifier. In that case, the amp should be serviced or replaced. If there is no hum at that point, the issue might involve the preamp or another ‘upstream’ source component. Some common causes of a ground loop hum are cable TV connections, digital recorders or satellite dish receivers.
If the hum issue is found to be with a cable TV coax wire, then a device called a MAGIC box possibly may help eliminate the ground loop from cable TV, or an OTA antenna. Some, if not all surge protectors have a cable or satellite in and out cable connection; try that as well.