Ground loop hum

I have been trying to help a friend resolve a strong ground hum issue with his system. Does anyone have any ideas that might help him?

Here is a letter from him outlining his latest efforts:

I did ground the Integra DTR-7 and Rotel RMB-1095 to the lower mounting screw of the wall outlet that the line conditioner is plugged in to. I found a 0.4 Volt difference between the RCA output grounds and unbalanced Rotel input grounds. This coincides with the voltage difference in cases to ground. After running the wire, the hum with volume down was reduce and the hum when turn up was greatly reduced. There was audio, woohoo!!!. At louder volumes the hum wasn't noticeable. I watch a move and it does get louder and is cleaner and more natural than the built-in amps for sure. I found out that I am not driving the Rotel good enough either. My Integra only puts out 200mV and the Rotel expects 1.5V. Anyway, there is still the slight hum and an annoying hum from the subwoofer. About a week later, I cleaned up the wiring and separated the power lines from the audio/video lines, but no difference. So, I disconnected everything and connected them one by one. I found the problem to be related to digital audio inputs with devices associated with digital video (Cable DVR, DVD). If I unhook the HDMI Connector, the hum goes away. The Mitsubishi Diamond line DLP only has a 2 prong plug and I looked for a place to ground it, but couldn't find any. With the DVD unhooked, I tried grounding the cable box, there was about .5 VAC difference there, even with the coax isolator, but that didn't make any difference. There are at least two options. They make HDMI isolators and digital audio isolators. I would think digital audio isolator would be less expensive and I would need 2. Oh, I would also need one S-Video isolator for surround setup that is use a few times a year. With the offending inputs disconnected, it is very quiet. Any additional suggestions?
Try removing the line conditioner, and any other surge suppressors or similar devices that any of the components may be plugged into. If possible, plug ALL components into the same ac outlet pair (using a power strip which does not include a surge suppressor, if necessary). Or, alternatively, plug them ALL into a single surge suppressor or conditioner, which in turn would plug into a single outlet.

At the very least, avoid having some of the components connected to an ac run that originates from a different circuit breaker than the other components are on.

It's not clear if the 0.4 volt difference in chassis potentials are with or without interconnect cables in place between those components. If with, it is strongly suggestive that the shield impedance of the interconnect cables is too high, and the hum might be reduced by using a better interconnect (or shorter interconnects). Actually, a better and/or shorter interconnect (resulting in a lower shield impedance) might help in either case.

See the following excellent reference, starting with Figure 1:

-- Al
Disconnect the cable TV coax and see if the hum goes away. If it does, buy a coax isolation device like the Mondial magic box (expensive) or the similar device from Radio Shack.