This one might be the problem you identified with the MG Head. I don't remember reading if you ever got that hum in another room of the house, or just where it is located. You originally stated that you went upstairs with the MG and the problem went away, but I don't remember if you said whether or not it came back.
When I had a hum in my RKV that went away it was a problem with the ICs, so I would definitely make sure all those are hooked up properly to both ends of your ICs. Mine were hooked up weirdly to my RKV, and the hum would get worse when I touched certain things, and better for others, and then came back to a normal hum when I let go.
Try this: run a long extension cord from another room on another breaker, and plug only the headphone amp into it. Did the problem go away? If so, it's a ground problem somewhere in your system. If it didn't go away, try taking the amp to another room of the house, but not the one where you plugged the ext. cord into. Did the problem go away? If so, again, ground loop or grounding problem in the room(s). If it didn't go away, you need to call an electrician. There may be a problem with your house's grounding system that could cause some pretty serious damage to your equipment since it might not be grounded properly when it needs it.
Another method would be to only unplug one component at a time, all the while listening for the hum in the amp. If you unplug something and the hum goes away, you've found the component introducing the hum.
do you have anything that runs intermittently on that circuit like a dehumidifier, sump pump, fridge, etc? If so, see if there is a correlation between when that item is on and the hum comes into your amp.
Man, let us know how things go. We want you to be happy and contentedly using your equipment, so let us know what you find as you go.