I had the cable company out and they hooked my Manley's up to a power inverter via there truck Vol-a ground/power hum gone.That was quite nice of them to do that. Use a floating AC source that does not have a reference to ground. What they did was mask the problem of their ground problem of their cable system. You have a ground loop problem caused by a difference of potential, voltage, between their cable ground and the ground of your homes AC Grounding Electrode System.
You can install a simple signal isolation transformer between your cable and your cable box. Check the archives here on Agon.