Dedicated line

10ga cryoed Romex from Vh Audio is hard to beat.
I would recommend solid core, as I have tried stranded before and there is no comparision, solid core wins hands down for dedicated lines!
I think 8ft grounding rod is code, but most electricians use 6ft because it is less work and gives up nothing compared to 8ft.
6ft is all you need.

12 ga is rated for a 20amp circuit, so you can use it with a 20 amp breaker.
10ga is better though IMHO!

You cannot ground your receptacles to a separate ground rod. In fact, no grounded conductors can be connected to a grounding connection on the load side of the main service disconnect. That means that the neutral wires of your dedicated circuits cannot be bonded to the subpanel's neutral bar unless that neutral bar is electrically isolated from the subpanel with a separate grounding conductor from the neutral bar to the main panel.

The reason you cannot run separate ground rods for branch circuits (or have more than one grounding point of your electrical service) is that a lightning strike will cause a voltage potential between the ground rods, resulting in damage to whatever is plugged into the receptacle. That's why Telco and CATV services are grounded at the same point as the electric service.

The mention in my post was unclear. I put in a new service panel for the entire house and drove two ten foot grounding rods which has helped my electrical issues. I think the prior posts agree with the ten gauge. The draw in my system was due to two Classe CA-150's being driven hard on a circuit which had other items such as lights and the front end. The reason for the ten gauge is also for future or current audio components that like power. My amps can demand up to 650 watts each which is a lot for a typical house circuit. I live in an area that is wet and prone to dirty electric. Hope this helps. Steve

this all sounds like over kill to me ...12awg or 10awg in real world applications would have no sound difference at all.