A couple other things that can cause hum are
cheap cable (or a cable gone bad),and speaker
cables tangled in power cables...

I disagree. You will not get audible hum from defective speaker cables.
You also will not get it from wrapping them around power cables. Defective
interconnect cables can allow noise to enter the system but the SNR on the
speaker cables -- after all of the amplification -- is so great that any
induced AC there will not be audible.

In all probability this is a ground loop problem.
--
Alarm and Home Automation System FAQ
http://www.bass-home.com/faq/masterfaq/faq.htm

Regards,
Robert

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