You probably have a ground loop. If you disconnect the cable feed to the TV set and the hum drops away that would confirm it. A ground loop is caused when there's a difference in ground potential between your cable and electrical grounds. You can solve it by breaking the ground connection at either end. On the cable side you can use a pair of baluns 75 ohm to 300 ohm & 300 ohm to 75 ohm wired back to back to lift the ground there. Or use a 3 prong to 2 prong ground lifter on the electrical side to break the connection there. Also make sure the preamp is not stacked directly on top of the power amp as it may pick up induced hum from the power transformer.
Buzz/hum through speakers
When I turn on my system, I get a buzz/hum through all the speakers. I have everything hooked through my Furman line conditioner. I have plugged my amp directly throught the wall outlet with only speakers attached to the amp and no noise. I have disconnected everything from the line conditioner one by one and only makes the noise when the pre and amp are turned on together. My dealer gave me something called Hum X and when I plugged the Furman into the Hum X, noise still there, it's only when I plug the Hum X to the back of the Furman and plug the amp into the Hum X, the noise is gone. Is there any way to get rid of the noise without using Hum X? If I can't get rid of the noise with Hum X, is Hum X holding back the full potential of the amp?