One problem that may arise with a HT setup is the fact that because there are so many components to power up, the system ends up on a couple of branch circuits. In my books, this is a no-no and it invites 2 different ground paths (because each ground has its own, theoretical level) which then invites all of the components that are interconnected with audio cables to have conflict with the ground path. If this is the cause (not a cable box), the solution could be to connect everything to one outlet (if the amperage will allow), so that there is only one ground point.
Wayner