I just set up a new subwoofer, the 8" Monoprice 8248. As soon as the power is turned on, it produces an audible, low-pitched buzzing sound. It isn't noticeable if the speakers are outputting sound, but it would very noticeable and quite distracting when the overall sound is supposed to be quiet, such as in silent scenes in movies.
The sub has a two-prong power connector, so it can't be grounded. The humming occurs even if I disconnect the line-in (it also has high-level clip connectors but I don't use them). Adjusting the volume and frequency knobs has no effect on the sound. If the sub is set to "Auto On" and it goes into standby mode, the buzzing goes away.
I've also tried plugging it into various outlets just to see what would happen. One unused outlet, two different kinds of power strips with surge protectors, and none of them made any difference.
So, what's going on? Is the sub defective? Does it need to break in? Is buzzing just something that happens in low-end subs? I doubt the latter - my ancient and super-cheap Logitech X-530 computer speakers has a buzz-free sub.