First, lets get one thing straight! There is no such thing as RF induced Hum. You have pretty much proved yourself that you have a ground loop problem. If I were to phatom a guess, your cable box and modem have two prong plugs and ground is being derived from the shields of their coax. This is not a good thing and is probably causing your problem. Good Luck!
RF Induced Humming??
I have cable TV and cable modem on one line. I can hear a humming sound coming from my speakers when my amp is on and nothing is being played. When I add a ground breaker to the cable coax at the jack in the wall the humming dissappears, but then I lose reception in my local TV stations and my cable modem loses connection. Does anyone have any solutions to this? Thanks.