Kaz,

It doesn't matter if your cable isn't hooked up to your amp. Take it from me. I was in the same spot as you. I coerced the cable company to come and install an isolator at the entrance and that took care of all my problems. It has to do with the way the cable company grounds the incoming cable.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated