1) Does the subwoofer amp hum with no input?
2) Does the receiver hum when the sub amp isn't connected?
3) When everything is connected, does the hum come from the sub, the receiver or both?
4) Why do you run a signal back from the sub to the receiver instead of just going one way?
5) Are you using an effects output like for an equalizer, that splits the preamp from the amp section of the receiver?
6) If so, have you tried leaving the straps in place, and using another preamp output instead, like perhaps a tape monitor output?