From what you've said, you have multiple sources of hum, but you keep leaving out critical pieces of information, so it's hard to be sure.
All of your a/v equipment, including your computer, subwoofer, TV, etc, must get its power from the same outlet. Otherwise you are likely to have hum. Sometimes you can get away with using separate outlets, but it depends on how power is distributed in your home and what else might be plugged into each circuit.
To localize the source(s) of the hum, you must unplug all other electronics from your receiver. There should be no hum. If there is, then you probably have a power supply problem in the receiver. Most likely a filter capacitor has failed.
Then start connecting one thing at a time. When the hum starts, that's your culprit. Make sure each addition is not connected to anything else when you plug it in, since it could be that secondary connection (like your cable modem) that is the real culprit. You have to be methodical about this, otherwise you'll just confuse yourself (and us).
If you cannot power the computer and its CRT from the same outlet with everything else, then you should use an optical connection.
If you hear hum from the PC when using only its local speakers and it is not connected to anything else, especially not the cable modem, then there is something wrong in your PC: most likely a filter capacitor has failed in the power supply.
I hope this helps a little.