Here's an update: turns out it's indeed a ground loop outside the guitar. Everything checks out on all three guitars. The ground loop issue isn't something I can remedy, as it's a building thing. Ebtech products do the trick. investigating now.

With these ground loop situations, the HD500 is known to produce this sort of hum, so I'm not alone.

Why do the other amps not have this problem? Can't speak for Marshall, but the lady at Peavey told me that the input jack grounds are lifted and diverted (probably to a transformer is my guess), thus completely avoiding ground loops in the electrical system. Mystery solved... Kinda. This is likely the reason why the HD500 runs silent when in the loop, as well.

Good to know that my handiwork wasn't to blame. Huge relief to know that Bill n Becky's gems are flawless. Now all I have to do is get my hands on some ebtech gear.