Any advice before I cart the receiver off to a shop is gratefully
accepted.

Try lifting the ground on the subwoofer. Use a cheater or a cheap extension
cord. If the hum disappears you have found the culprit. If not, ask and
we'll make more suggestions, but this is the first (and simplest) thing to
check.

Alarm and Home Automation System FAQ
http://www.bass-home.com/faq/masterfaq/faq.htm

Regards,
Robert

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