Help with speaker buzz

Last month I purchased a new pair of subwoofer cables, I sent the cables back to the manufacturer for re-termination to RCA due to I purchased a new preamp that did not use XLR. The system played fine with the new cables in, however whenever I turned the preamp off I had a very loud speaker buzz and sub hum. I went through a process of chasing a ground loop with the technicians of the company, finally I had to send the cables back for repair, I inserted my old sub cables and the system was dead quiet on or off. Upon return of the new sub cables from repair I put in the cables played music all was well, however I turned off the preamp with amps on and the buzz came back but this time the buzz was much quieter and no sub hum this time around, I unhooked the active shield of the cable and the buzz went away. I failed to mention these are the top of the line Synergistic Sub cables. Can anyone help me diagnose this problem, has anyone out there experienced the same sort of problem. I am definitely at a lost. My system is on dedicated circuits however my subs are on house line, no cable tv or home theatre in my dedicated listening room if that helps any. I also disconnected all my direct tv sources throughout the house to see if I was getting feedback from a ground loop. I really like the sub cables, however I am on the verge of asking for my money back. Please Help

Thanks
Mitch