My subwoofer hums even when nothing is playing — what causes it?

Almost always a ground loop: two connected devices sitting at slightly different ground potentials, and the difference riding into the sub through the signal cable as a low hum. The sub is usually the messenger, not the culprit.

The diagnostic path, in order, because it tells you where the loop lives.

First, disconnect the signal cable at the sub with the sub still powered. Hum stops? It is a loop somewhere upstream, not the sub. Hum continues with nothing connected? Skip to the end.

Second, power the sub and the AVR from the same board or extension. Two devices on two different wall sockets, on different circuits, is the classic Indian flat arrangement and the classic cause. Sharing one point of earth often ends the story.

Third, look for a cable-TV or dish connection anywhere in the chain. A set-top box coax earthed to the building somewhere far away, feeding a TV, feeding an AVR over HDMI, feeding the sub, is a loop drawn on a diagram. Disconnect the coax and listen. If the hum stops, an isolator on that coax line is the fix.

Fourth, route the sub cable away from power cables and avoid coiling the excess next to a transformer or a UPS. Induced hum is rarer than a loop but it exists.

Fifth, still there? An isolation transformer on the offending signal line settles most stubborn cases cleanly. Do not lift the earth pin on anything to make hum go away; that trades a hum for a safety hazard.

And the case where hum continues with the cable disconnected: the sub's own plate amplifier is the problem, and that is a service call rather than a wiring one.

The one thing I would not do is live with it. Hum is a symptom, not a personality trait, and it will not get quieter as the sub gets older. Send us the connection chain, ideally as a photo of the back of the AVR and the sub, and we can usually point at the loop from here.