Electrician dogma requires 20A circuit use 12ga wire. Good luck getting them to use 10ga especially if there's an inspection involved. But yes, it's legal and no, it won't make as big a difference as using JPS romex or even better using a power regenerator.
12AWG is the minimum size permitted in a 20A circuit. Some 20A circuit require larger than 12AWG.
I would stay far away from a regenerator!
A dedicated circuit implies a separate ground from the single outlet to the service panel, using the ground conductor in the romex.
a] the Safety Ground is one conductor in the Romex®. A dedicated circuit DOES NOT implies a separate ground from the single outlet to the service panel.
A dedicated circuit is one where you have control of what loads are connected to it.
If you add another outlet to the circuit then it's not a dedicated circuit anymore, it's shared, and both outlets will use the same ground back to the panel.
NO, a dedicated circuit may have as many receptacles has you wish.
Two true dedicated circuits will require two separate lengths of romex back to the panel. This also violates electrician dogma and they will be hesitant to do it because it looks wasteful and stupid to the inspector. But it's legal of course.
A dedicated system can have a dedicated feeder to a small breaker box then several dedicted circuits.
If by "separate ground" you mean an additional earthing rod for audio system only, that technically does not meet the national electrical code. A building can only have one earthing rod for the sake of preventing a fire in case of a lightning strike.
This is vary, vary TRUE.
One building, one Earthing Rod system.
No matter how great the wire or using a dedicated line, the utility signal will still have clipping distortion (1-3% THD) because appliances that rectify the line to DC (electronics) only draw current at the voltage peaks. Only a power regenerator can fix that by regenerating the power from DC. It's a bigger improvement than a dedicated line, imo.
Good luck with your project!
Don't worry about utility THD, your power amps are the biggest offenders not appliances.
Once again, stay away from regenerators.