FM interference with AR sub

I have an Acoustic Research powered sub (model 108) that works fine except for one small problem. When connected to a reciever (via speaker inputs since no preamp out available) it causes severe interference with FM reception. Apparently some kind of hash is running up the speaker cables into the amp (or possibly radiating from connecting wires). I have tried using sheilded cable and zip cord and both give the same effect. Maybe the sub uses a switch mode power supply (yuk!).

In any case would it be possible that using an audio input/isolation transformer on the the interconnect could help? Any source for reasonably priced transformers for this purpose?