Those are pretty much all the amps I don't use a whole lot - occasionally yes, but not a lot.   Some of those amps do hum in real life, but your issue seems to be external to the POD as gating the input gets rid of the buzz for you.  What you could do to ensure your guitar gets a good ground is to connect one of the HD500's output jacks to an amp that is grounded by being plugged into the wall but not switched on, as there can be grounding issues caused by using balanced XLR cables that are wired with the cable 'shield' braid connected to the casing of the XLR plugs at both ends. Not all XLR cables have the little link connection soldered in place, but mic cables tend to because of electrical safety concerns when handling a microphone.  Having the XLR plug casings connected to the cable shield wire at both ends can defeat whatever the XLR Ground Lift switch is set to on the POD as you don't have true isolation of the balanced wiring that way and hum can occur.  I'm not encouraging you to snip any links in your XLR cables here - that's up to you as long as you keep things electrically safe at all times by other means if necessary, but rather trying to explain (and maybe not too clearly - sorry) what might be contributing to your hum/buzz issue.  Have you tried just using a standard 1/4" output to connect the POD to your PA instead of XLR? You might find that passes a better ground for your guitar than the XLR route is doing.