The problem is that the cable service isn't grounded quite the way it is supposed to be (as you might have guessed from the 7700's manual, it's quite common). You can tell the cable company that you have a ground loop hum coming from your speakers, that you've determined that it's being caused by your cable service (specifically, unhooking the cable from the box causes the hum to disappear), and that you would like them to provide a ground loop isolator to resolve the problem. Odds are probably 50-50 that the teir 1 customer service rep will understand the issue, but the service guy they send out should be familiar with the issue.