You may experience a "ground" differential problem using different AC circuits.It is also possible that this may arise from the quality of grounding in the two cables.Had you tried "floating" the earth-ground on the humming culprit?
IC's and Humm
I hooked up another amplifier that I added to my system. My preamp has two pair of outputs. Before my system is dead quiet. The new amplifier runs quiet as I tried it running my main speakers. But when I try it with a secondary small speakers I get humm....so I tried switching the IC's from my monoblocks(NBS) to this stereo amp which was connected using some ($100) Monster IC's. The NBS are dead quiet and the Monsters produce a low audible hum. When using the monsters the humm also increases slightly when I turn the lights on. I think the problem is that I have run out of dedicated ports for this amp....so I use a nearby regular port. Sorry for the long description but I'm wondering is it just the IC or should I have a couple more dedicated ports installed. I run 2 pairs of monoblocks,preamp,dac,transport....and to get it dead quiet is like a combination to a safe....it must have a certain layout or I get humm and my tube amps are realy finicky about it.