The very last thing I would suspect is the wiring. It's an inanimate object.

In addition to what Sam said:

Disconnect the amp input from the system. Do you hear hum? > replace amp.

Listen to system output with your butt set while disconnected from the amp.

If you are going to do this kind of work and charge for it you should have the proper test equipment. A signal generator and spare amp with speaker (and knowing how to troubleshoot with them) will find the problem in minutes.

-Hal





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