I can agree that there may have been (or still is) a "grounding problem". Nonetheless... why is it so much of a probelm to call the electrical contractor to evaluate the situation?
For the same reason that when you call tech support for anything related to your computer or Internet, they ask you to reboot your machine. It is busy work unrelated to the nature and cause of the problem.
I have tried to explain this to you but you seem unwilling to listen. So I will be more specific but holding little hope that it will make any headway.
The word "ground" in ground loop has nothing, let me repeat, nothing to do with the ground coming into your building. It refers to ground pin of the input on your audio equipment. It is that ground, which if at a different level than the source equipment, will cause the hum or "ground loop."
The ground in your breaker panel is there to shunt external surges (like the infamous "lighting") to the ground. It is also there as the third leg in your outlet as to eliminate the safety risk of your metal encased equipment becoming energized, causing a potentially lethal shock. Its purpose is not to reduce noise, get rid of hum, etc. no matter how much you want it to be associated with that. OP's house has the right ground connection for its intended purpose. Safety.
There is a complex connection between your electrical system and how consumer audio gear works that can cause hum to all of a sudden appear. Or disappear. Without instrumentation you will not know why is there.
Also... why would a competent/licenced electrical contractor install 3 grounds? Are they bonded?
He can put a thousand grounds there. It has nothing to do with the topic at hand. Go and hang out in electrician and building inspector forums and ponder about it there. As I explained, there is only one ground in the building which is the requirement for safety.
Also... no one asked about the wiring the home. Is it all copper romex? Is there aluminum wiring? Is there knob and tube...,..... etc.
You can have systems with no hum using any kind of wiring. If you think otherwise, you need to explain.
Anyone can mask a problem with gadgets, but let's get to the potential issues that may need to be addressed.
No. Putting a transformer does not "mask" the problem. It removes it. It isolates the ground connection between two pieces of gear as it should have been day one in consumer audio. Equipment should tolerate differing ground potentials but depending on the design, it may not do so or do a borderline job. Since you can't change the design of your equipment or keep rewiring the building hoping you get lucky, the transformer provides an appropriate solution. If you don't like that option, sell the gear and buy something else that works better.
Everyone has stated over the years that using a 3 prong to 2 prong adapter is not recommended, but... no one questions the competency of the "electrician" that replaced a "fuse panel" with a "breaker panel" and may not have properly grounded the new work?
Once again, OP used an outlet tester and showed correct wiring. Nothing else in his home is having a problem. You can imagine all that you want about this situation but that is not how we assess and solve problems like this.
Today: MY house exists
tomorrow: MY house has burned down
If having audio hum causes safety issues that cause fire, it would be in the code.
You are barking up the wrong tree Rat. Countless people have proper and safe wiring, buy a new piece of equipment, get cable, dish service, etc. and get hum. Then move wires around, or some other random thing and hum gets less loud, louder, or goes away. All again, in a home that is wired safely.
EDIT:
Unless I missed a post, the OP never stated whether the "electrician" was licenced nor whether permits obtained. Anyone can buy the materials to change out a fuse box and replace with a breaker box and drive a rod into the earth. That doesn't necessarily denote that the work has been performed properly and according to code.
Yup. And a drunk sailor may have made the circuit breakers in your home so let's worry about it catching on fire too....