I know it's been some time and it doesn't look like a solution was found, but I had the same problem with my B52 at100 head. In scouring the internet, I came across a random forum where someone found a solution for this. For those using a TRS cable, you can open each end of the cable and desolder the sleeve wire from the sleeve of the connection. I think it has something to do with the fact that the sleeve connection from the amp would power the lights on the amp's included footswitch, so there's a small electrical current flowing through the sleeve, and that translates to hum when connected to the hx effects amp control input (I'm a dunce when it comes to electrical, so I could be wrong on this). Disconnecting the sleeve wire in the cable got rid of the hum for me and I haven't had any electrical issues stemming from that, so give that a try.