Two things that might cause your problem...
If the branch circuit home runs are long you may have a ground loop problem. How far would you say they are from the receptacles to the main electrical panel? Try connecting the three equipment grounding conductors together at the receptacle roughin boxes . This will create one common star ground point close to your equipment.
Second problem could be, if I understand you correctly, you had three new dedicated circuits installed. Each with a seperate 20 amp single pole breaker. I am just guessing but I believe the electrician put one circuit on "A" phase L1, Next circuit on "B" phase L2 and the last circuit on L1 or L2. Have the electrician put them all on the same phase. Either L1 or L2, but not both. You could put your power amp, the #10 wire conductor on one 20amp single pole breaker and the other two circuits, the #12 wire conductors on one 20 amp single pole breaker. Just make sure they are both on the same phase. I would try this first before the ground fix.
One thing more, make sure the electrician connected the equipment grounding conductors to the neutral/grounding bar in your main electrical panel. This method is for a house main service panel. The neutral is bonded to ground by the grounding electrode conductor on this bar. All branch circuit neutrals and equipment grounding conductors terminate on this bar.
Hope this will help,Jim