Aging a ground to that was not part of the contract. That is why I am now considering having a ground added.
@mschott FWIW dept.: in order to add a circuit of any kind, whatever it is connected to has to be up to code. So either the electrician did something illegal or else found a way to provide a legal ground.
The suggestions to try the amp elsewhere or with nothing connected to its inputs are good ones! It may simply be that the amp has a hum problem; one potential source of a hum in an amplifier is filter capacitors failing in the power supply of the amp. That can lead to power transformer failure which can be really expensive and disheartening. So its worth it to sort out what is going on.