First off keep in mind electronics have some faint hum, no matter what you do, so you may never be able to eliminate ALL of it if you analyze these things by walking over to your speakers (and sub) and putting your ear right
up to it.
Ground loop hum can be a real bear to diagnose because it usually isn't just one connection's fault but rather the system seen as a whole. Even though , let's say for example the problem seems to go away when the incoming RF feed to the sat box is severed [try that by the way], that same "problem" doesn't exist if the sub is disconnected. See it's really the system
configuration not the component itself.
Keep in mind what may seem like perfectly innocuous connections can be the "culprits" even if they seem far away and unrelated, even remote trigger wires, IR blasters, etc. It may be that unplugging the power to the sub [or temporarily putting a cheater plug on it to break the AC ground] makes the problem in [perhaps] the front left speaker go away, for example.
In general one important rule is to use a "star grounding system topology", i.e. all grounds start at one point, say one beefy AC strip plugged to one AC outlet, and then everything
emanates out of that like the rays of light coming out from the center of a star. Add a new
outlet in the room, like for that far away display or sub, and you break the rule!
All you can ultimately do is unplug literally everything and keep adding components one by one until you find what new connection suddenly makes the system have a new secondary ground potential which causes the hum. Then work from there by adding a transformer or isolator box, changing the audio path to that part optically instead of through a wire [often done when subs are the problem], and using various other band aid approaches.
Read , more here. good luck:http://www.rane.com/note110.html