Dedicated mains supply unexpected result

Aluminum is for cans! Copper is for electrical wire and pennies.

As an Electrician, I can say cable size increases with the demand on a new house service. The old houses were 70 amp, then the standard was 100 amp, now we are at 200 amp.That is a lot of load, at the same time the houses are becoming mansions and the demand is there for a large residential service. Huge panel boxes are installed due to code changes, damn near everything thing gets it's own breaker.

One thing that never changes is ohm's law. Fact a larger conductor has less resistance and allows more current to flow causing less heat on the conductor. The same amperage on a smaller conductor causes heat and breaks down the insulation causing fires.

Residential installations are  considered a single phase setup, 2 hot conductors a neutral wire and a ground wire. By adding new larger cable, that should not have cause any issues to your audio system in any way. At that position the new cable is most likely a better copper, and has way better insulation.

The neutral and ground conductors should be isolated from each other throughout the entire house the come back together in the breaker box, they are bonded there. Depending on how old your main breaker box is, there may have been something the electrician had to do to get the larger conductors to terminate. The new cables may not have fit in the original lugs, he may have had to change them? That alone should not have caused any issues. 

I assume that the work done was more than just a wire replacement but that you also had a new panel installed?

Get something like the Quille plug in voltage tester (9 pounds) on Amazon and watch it while playing music.  Pay attention to the Neutral to Earth as well as the Neutral to Hot voltages.

It could very well be a noise issue caused internally.  Try turning off every other circuit in the home to attempt to localize the problem.

Send pictures of your mains box open so I can see how its landed/wired

Per Eric, did they change the mains box.

Did they change your main breaker or add a AFCI.

If we are talking about a tube amp, check/reset the bias.