Sunn 1200s/Fender Bassman 1200 effects loop hum

Hello folks, first time poster here. My apologies if this is not where I should ask this question, but I wanted a trustworthy opinion and…well…who better to ask, right.

I own a Sunn 1200s bass amp, (Sunn 1200s/ Fender 1200 Bassman from the mid 90’s) I would like to use my bass amp's effects loop for a rack effects unit (lexicon MPX-200). The amp is dead quiet until I plug something (anything) in to the effects loop return. When something is plugged in, I get a steady hum, like an AC hum. The hum is the same volume no matter where I set the amps volume. It hums with or without an instrument plugged in, and it doesn't matter what is plugged into the loop, still hums. I even tried a simple patch cable between the loop’s send and return and still, it hums (tried several different cables to be sure it wasn’t a cable issue also). When I unplug from the effects loop, it's back to being dead quiet......Also, I have a rack mount line 6 bass POD pro. I have tried running the bass directly into the POD then from the POD out to the Effects loop return (directly to the power amp bypassing the preamp altogether) and I get this same hum. I have tried several configurations on several different power systems (different buildings) and still, the same hum persists. Could I have a grounding issue inside the amp only affecting the loop? What do you guys think? Any ideas/suggestions would be greatly appreciated. Thanks in advance for your time.