Common problem when you attach TV w/cable to audio equipment. Had the same problem and it turned out to be grounding of the cable. When the cable was physically disconnected from the system, hum went away. When it was connected, it came back.
I tried all kinds of grounding for the cable, but nothing worked that well. You can get isolation devices that break the DC/shield of the cable coax (with a capacitor), but they don't often work.
What finally worked for me was using a transformer isolation device for the TV audio. It is made by ART and has two channels transformer isolated. It has both single ended and balanced inputs/outputs and is used to break ground loops for professional sound set ups. I connected the audio out from the TV (or cable box) to the ART transformer and then to the audio system.
Later when I switched to Direct TV (satellite), I had them float the dish which I grounded to my audio system ground. No problems with that setup.
Cable is tricky because the cable shield may be grounded at one end of your house to a different physical ground than the circuit ground of your audio system. Re-gounding it at your audio system may make it worse because then you have a ground loop.