Hum in Speakers with New Dedicated 20 amp Circuit

Call a licensed electrician. My brother, the E.R. nurse, makes his living off of amateur electricians and guys whose last words were "Hold my beer and watch this".

Make sure the sparky who comes to your home is the license holder & not an unsupervised apprentice or journeyman, as is often the case here in D.C.

As others have said, after turning off power to sub-panel AT MAIN PANEL, run the bare copper ground to the bus on the left of the panel and attach all grounds to this bus. Also make sure there is no bond strap between neutral and ground bus (the big twisted bare wires?). 

This is because if you lose the neutral between the two panels for any reason and the ground is attached or bonded to neutral bus, the ground wires back to the assorted circuits could then become hot. This only applies to sub-panels, not to main panels where the ground and the service connect are a short distance (hopefully) from the panel. 

Also, you said the wire is 12-2. Either you meant 12-2 w/ ground, 12-3 or you have no ground with your new circuit. Plus remember the red wire is hot along with the black for your combined incoming 240 VAC. 

@ieales ...*L*  What to run?

Whatever boats his floats.... ;)

At 60Hz it's clearly an earth loop.  These can be frustrating to locate.

1.   Plug a completely different system into the outlet you are using.  Is the hum still there?  Then it's the house supply.

2.   Strip down your system as far as possible and see if the hum is still there.  If so, substitute items in turn with outside items, including power cords, connecting leads.  Eventually the hum will be absent and you will have found the offending item.

If his sub panel was installed years ago, isolating neutral did not apply at the time.