I had this problem, I heard a radio station from my center speaker... it turned out that the interconnect that was going to my center was the only one that wasn't shielded... So, after I replaced it the center sounded perfect, only playing what I'd selected

For the center speaker I'd used a diy silver interconnect that was Chris Van H. (don't remember how to spell his last name) design.. it was just 2 silver wires wrapped parallel to each other and only about 10" long.... so... length may not matter all that much if they're unshielded...