Hum when connecting HXFX ext amp to Friedman Runt 20

[ledvedder]

Hi guys,
I've been using my HXFX in 4cm to my Runt 20 with no issues. Tonight, I decided to give the ext amp a try to change channels on the Runt. As soon as I plugged the cable from the HXFX ext amp jack to the Runt's footswitch jack, I started getting a hum. I'm assuming it's a ground loop? I tried switching the ground switch on the Runt, but still hums. This works perfectly fine on my DSL and 5153. Any ideas how to resolve it with the Runt?

 

[ledvedder]

Removing the ground from the HXFX side of the switching cable removed the hum. But there's still an audible pop when changing channels.