This seems different from other hum-related posts, so I'm going to ask. Is it normal for the F15HP to make a faint constant hum? Is this just from the sub's amp? It can only be heard when the room is quiet, with no music or movies playing. (Why is there no music playing, you are probably asking? I sometimes, although rarely, just read in silence. )

The hum occurs whenever the sub is plugged in. It occurs regardless of the power switch position - off/auto/on. It is also present regardless of which electrical outlet the sub is plugged into.

The hum is present whether the rca cable is connected or not.

The level of the hum is constant and doesn't change when the sub volume is adjusted. It also doesn't get louder when the AV receiver volume is turned up. It doesn't seem to be coming from the driver, and is not noticeable when listening to music or soundtracks.

So, is this just a normal sound of the sub's amp? I can certainly live with it, but would like to know so I can quit worrying. Thanks!