I had taken a couple of the pieces down to my local hifi shop. We plugged them in, no hum. And that was straight into the wall socket, no filtering of any kind. I just happen to be cursed with bad ac into my home. I will take all suggestions here, apply them, and hope for something to work.
Did you have the electrician check the mains voltage measurements as I asked?
To high of a mains voltage can cause transformer to hum louder than normal.
As Atmasphere said in his post odd harmonics can also cause a transformer to hum louder than normal. Where your BPT 3.5 isolation transformer unit will not pass DC on the mains from the primary winding to the secondary winding it cannot block odd harmonics from the primary to the secondary.
You need to first isolate the things in your home that may be causing the problem.
Quickest way to do so is turn off every branch circuit breaker at the electrical panel except the 20 amp dedicated circuit feeding your audio equipment.
****Caution**** If the electrical panel is a bulldog or ITE pushmatic panel where you push in to turn off the breaker and then push it again to reset do not perform this test. Pushmatic breakers are notorious for not being able to reset.
For what it's worth most odd harmonic problems are caused by electronic devices within the home. Anything with a microprocessor such as a refrigerator, microwave, dishwasher, washing machine, cloth dryer, furnace, ect. Variable freq drive on a furnace can cause big time harmonics back on the mains.
Dimmers are terrible. They can even radiate RF through the air several feet, not to mention on the wiring in the home.
Check this video out.>https://www.youtube.com/watch?v=OCK5W9vlAE0
CFL and LED lights that screw into a regular 120V socket have an electronic transformer that put crap back on the mains. Do you have a lot of them in your home?
The list just goes on and on. But many people have these device in their homes and are not experiencing the problems you are.
Sorry for the detour, back to the test.....
Plug in just one piece of audio equipment directly into the wall receptacle outlet and check for the louder than normal hum from the power transformer of the piece of equipment. Best to pick one of the piece's of equipment you took down to the local hifi shop.
If no abnormal hum, start to turn on one breaker in the electrical panel at a time. Check the piece of audio equipment each time after a breaker has been turned back on. Continue until you find the circuit that is feeding the culprit causing the problem. Next check the loads that are connected to that circuit. Isolate the problem.
IF, with all the breakers turned off you still have the louder than normal hum go outside and look up at the utility transformer that is feeding your house. You should be able to follow the secondary side wires of the transformer and see what others houses are fed by the same transformer.
Are they just houses? Any small commercial or garage businesses? Are you fairly close to any commercial or industrial facilities?
If just houses, no businesses, do you know any of the neighbors that are fed from the same utility transformer as your house? If so see if they will let you plug the piece of audio equipment in a wall outlet in their home. Check for any abnormal Hum? No abnormal hum? If no abnormal hum your problem is somewhere after the power leaves the transformer and your house.
If you take the time to do the tests I outlined, post back your findings.
Here is some stuff I found searching the Net. You will have to copy and paste. For some reason the Agon [url] xyz [/url] thingy wouldn't work right.