@ tomiiv30 OP
I decided to deleted my previous post because I didn’t think it was right for me to instruct/dictate how the electrician, you hired, should go about trouble shooting for possible problems on the electrical service and or wiring of your home.
I did save the first part of my post about the two ground rods that were installed for the equipment grounding of the two new dedicated circuits.
I would appreciate it though if you would note and post back what the Line 1, leg, to Line 2, leg, voltages are as well as Line 1 to neutral and Line 2 to neutral voltages are at the electrical service panel.
To high of an over voltage above the voltage rating of the torid transformer in the amp could also add to the cause for the transformer to buzz louder than it normally would.
@ tomiiv30 OP
Make sure you ask the electrician exactly what he did with the equipment grounding conductors for the new dedicated branch circuits he installed. TAKE NOTES! A picture is worth a 1000 words. Have him physically show you exactly what he did.
It is possible the electrician wired the new ground rods per NEC 250.54 ...... I’ll give the guy the benefit of the doubt.
If he did wire the 2 ground rods per NEC 250.54 "Auxiliary Grounding Electrodes", then the equipment grounding conductors for each IG receptacle are connected to the branch circuit wiring equipment grounding conductor and the equipment grounding conductor of the branch circuit is connected to the equipment ground bar in the electrical panel the branch circuits are fed from.
Per NEC 250.54 a ground wire is installed from the ground rod and is connected to the branch circuit equipment grounding conductor. He can use any any size wire he wants to use from the ground rod to the branch circuit equipment grounding conductor. Hell, it can be 20 gauge.
Does an Auxiliary Grounding Electrode really do anything to improve the sound of an audio system? It can act as an antenna and add noise. It also provides another path for a lightning high voltage transient to enter your home. Lightning loves aux grounding electrodes.