I think this is where I trot out a story I've previously posted around the interwebs, so if you're hearing the spouse tell the same joke for the umpteenth time, apologies in advance.
I bought a Hot Rod Deville off CL in Boston from a nice guy. Got it home - hum city. Swapped guitars, amps, cords, etc. etc. etc. and came to same conclusion: it's the HRD. Guy was super nice and let me return it, expressed surprise, etc. I swore up and down that nope, I tried everything and sir, your amp has an electrical leak.
Two weeks later, bought a new HRD. Get it home and ... you know where this is going: hum. Go through same motions - nope, HRD hums. Since I'm smarter than Sherlock Holmes, cleverly deduce that there is some piece / part in HRD's that hates the rest of my then-rig(s). Feel smug; decide to return amp to GC and save myself trouble by getting something else.
Digression: I'm very gentle with gear (I think this comes from working on punk/indie tours where I saw gear take too much abuse). I use standby switches, place things gently, etc. Ergo, I always turn an amp on/off before moving it. No sense in not doing it, since I'm not on stage, and why risk dropping it with the tubes glowing? etc. etc. etc.
As I walked towards the humming HRD, about to turn it off and pack it up to go back, a girl (I'd been chasing) called. In my urgency to not miss her call, I brain-locked and picked up the HRD and turned and answered the phone, amp in hand. After speaking with her and hanging up, I looked down and noticed a red light - and no hum whatsoever. By turning that sucker 90 degrees, I eliminated all hum.
Moral: electricity is a dynamic situation. Before you go "solving" a problem, recreate it predictably in several settings. Just saying.