Hi Dave,
The symptoms you describe look like 'radio-interference' caused by light-dimming equipment. The switching-technology generates many harmonics that come into your music equipment via the mains leads. A proper 'low-pass' filter could solve this problem.

Another possibility :
Your equipment is powered by different main-voltage groups. In this case it might happen the Earthing potential is slightly different and current flows through the earthing leads.

I hope this helps,

Roel