subwoofer "hum"

How loud is it?  Do you only hear it when you put your ear to the sub or do you hear it while in your seat?  You may have a ground loop if it is loud.  You can swap cables to see if that helps.  I use a 3 prong to 2 prong adapter on my sub amp's electrical cord.  Eliminated the hum for 75 cents.

You should not be able to hear sub-woofer hum at all. If you do, there may be one of three problems, two of which you can address.  The first has to do with the polarity of the power line. If the plug is two-pronged and both are the same size, then you can try to reverse it in the wall jack and see if that has an effect.  If the plug is two-pronged, but one prong is wider than the other, you can't reverse it and it is polarized already, and so polarity may not be the issue. If the plug is three-pronged with a ground, then again you can't reverse it. The second has to do with the grounding between your AVR and the sub-woofer. You probably are running an unbalanced interconnect that has an RCA (phono) plug on each end. Often, an insufficient ground is set up between the two when an unbalanced interconnect is used. So, just as with a turntable, you may find that by running a ground wire between the electrical chaises of the sub woofer and the chaises of the AVR. If neither of these two situations exist, then your hum may be coming from a defect in the circuitry of the sub and in that case you'll need to take the box in for repairs.  Make sure that the repair person finds the same problems you have - in this case hears the same hum.