...
Disconnect as much stuff as possible from the receiver. Minimal or no input connections - Play an FM radio for sub-woofer testing
I'll try that, but I only have two inputs as it is (Tivo and Apple TV). I guess it's possible that this is coming in over the coax on the Tivo...
Apple TV and Tivo would also bring the TV into the equation. Everything in a connection chain can cause or link ground loop noise.
If the ground loop isolator has a spec for frequency response, it may roll off into the lower Hz range. It may also have an insertion loss dB spec, perhaps even a frequency chart.
Generally speaking, connecting
portions of the linked equipment to
other/distant ground points won’t fix ground loops. Often make it worse.
Electricity
always travels in a loop, including induced ‘noise’ current from ground point voltage differentials. The key is to find where the different ground point voltage potentials are located and somehow break the circuit loop through your equipment.