Musicians Roadhouse

I'm getting a hum and here is the setup. I've been playing with two amps for years. I just recently picked up a Marshall Vintage/Modern. When I plug in the two amp setup with the Marshall, I get this loud hum. I swapped out the Marshall for another amp....NO HUM. What's up with this? I've checked all cables and made sure they are plugged in to the same power supply to avoid a ground loop. All tubes are good. And, I've also taken out pedals that are in the path.
Any suggestions would be greatly appreciated.

You need a transformer isolator on the guitar signal into at least one of the amps.
There's a lot of myth and bullshit being promulgated about earth loops..... "Plug the amps into the same outlet", "Use isolated power supplies", etc.
These methods can help eliminate *other* sources of hum and noise, but they will NOT eliminate a ground loop.
You MUST break the path to ground on at least one of the *signals* into one of the amps.
Note: I said SIGNAL ground, NOT mains power ground!
NEVER cut the mains ground!!!!!! You will create a LETHAL situation!
Isolate the path to ground by inserting a transformer isolator in the signal path.
Don't use cheap isolators - they are NOT all created equal.