Oh, wow, yeah, that's really interesting. I don't understand electricity or know nearly enough about electrical engineering to be sure I understand the effect or flow you're describing, but if I (dimly) grasp what you're saying, it would explain exactly the behavior I observed.

Grounding really is incredibly weird (and, again, I say this as someone who is shamefully ignorant of electrical principles). It's no surprise that some 'audiophiles' become so superstitious about electricity. Its behavior in a stereo can be mysterious. Just looking at an amp funny seems like enough to cause a ground loop.