One suggestion: Get a couple of the clamp-on ferrite core chokes, and put them on the speaker wires, near the amp or receiver. An extra wrap of the wire through the core will increase the effectiveness of the filter.

Often, the output transistors will "detect", or rectify, strong RF signals picked up by extended speaker wires, which act as antennae, and the amp then amplifies the signal. The choke shunts the high frequencies.

Larry
www.fineelectricco.com