On a very practical note from personal experience... I had a hum problem whenever I attached the cable tv coax cable to the system. If I removed the cable tv connection, the hum was gone. I was able to eliminate the problem by wrapping bare copper wire around the outer conductor of the "F" connector plug from the cable tv coax cable and grounding it to the center screw that holds the cover plate onto the AC wall outlet where my gear was plugged in. This eliminated or greatly reduced a ground-loop problem and the hum was gone.
Also on a practical note... if you can plug all your gear into ONE wall outlet, you're better off than plugging it into different outlets. Plugging into one outlet ensures one point of ground for your gear and minimizes ground-loops which can cause hum.
