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Shashi,

You might want to have an electrician out to your house to help trace this and figure out what the problem is. You may have a poor wiring connection, i.e. corroded or oxidized contacts (acting as a semi-conductor or spark gap), that is causing 60Hz radiation as well as HF noise. Years ago, someone from Pepco knocked on my door to let me know that he was driving by with an RF monitor that they use (presumably for detecting power line issues), and was noticing excessive noise or sparking indicative of a bad wiring connection in my house. He explained that this is frequently a cause of electrical fires, so wanted to warn me. I suspect that your probe and speaker cable may be acting as an antenna picking up a similar issue. I could be wrong and it is unrelated, but having someone who knows what they are doing to help trace this down is probably a good thing.

David

This is the noise floor measured by REW.
Look at the 60Hz fundamental and its harmonics. Any ideas?

I think you just have a ground loop. You have a very complex system setup and its going to be very tedious to hunt down problem.
You got to simplify and just start with one amp and one speaker.

Did you try David's suggestion?

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Problem is identified and mostly solved.

Replacing the dimmer switches with standard on/off switches reduced the hum picked up by the probe significantly.
Second, turned off the wine cooler in the basement.

With the David McGown's help, I was able to eliminate the hum in couple of amps. There is still a slight hum in one sub channel coming powered by a cheap solid-state amp. Going to replace the amp soon.