Xlr to rca

Hello, I am trying to hook up a large bgm model 620 b pro amp to my stereo, it only has 1/4" jacks, it was given to me with an 1/4" to rca 1.5m cable, I've tried hooking this up to a luxman stereo reciever and to a McCormack tlc-1 pre amp both of which have two sets of pre outs, I cannot have the outs connected to my velodyne hts15 sub and to the bfn amp at same time without mad hum, if I disconnect the sub or the bmg rca either one all the hum goes away the hum is present in both the main speakers on bgm and in the sub, the cable is wired from center pole of 1/4" to center pole of rca I was wondering if there is a pole difference between old pro audio and home audio, should I flip it so it center pole to outer and vise versa? I've tried various grounding options with no luck, but it sounds like a ground loop. Any thoughts on how to fix?