SWR 550x effects loop pops awful

[COOL AND DEADLY]

My SWR 550x head is good enough for me. I like everything about it except that when I use the effects loop I get terrible popping noises. I've checked the cords, pedals, power supplies, etc. no luck. I have been just running my bass directly through the bass into the input. It works fine, but what's the point of the effects loop if I can't use it. Anyone else have similar problems? Any advice? Thanks TB'rs you never let me down!