Ok Dave this seems a bit more complex that first thought. Not sure now how the cable's ground hum is getting into your rig, as it sounds like they're isolated from each other, but apparently they are not (unless of course as in when you connect the VCR to the preamp, that's an obvious path).
Cable systems are widely notorious for these problems, as their ground's potential isn't normally the same as the ground in your house. One other thing that I recall doing was connecting a small 20awg. stranded insulated wire from a wall outlet's ground screw to the Cable coax's ground sleeve. I wrapped about 2"to3" of stripped bare wire around the F connector's sleeve & fastened it with a wide nylon cable tie. This achieves a common potential of house ground to cable ground, & may or may not help you but it's an easy experiment & almost free if it works. If any of your AC plugs are reversible you could try playing around with that as well. I'm still not sure if I have your whole picture in mind so if I've missed something then do elaborate as I (or someone else) may think of other alternatives. Usually the hum-bucker is suggested, but looks like that caused you some other problems.
Thanks BTW for your vote of confidence; I try to help others here as they've helped me too. I'm willing to discuss this via phone too, if you're otherwise unable to find a fix just email be via the hyperlink so I can get the "whole picture". Good luck!