Phil: Putting the sub breaker box in the studio or very near the studio is a big advantage. It’s a central point, all runs are short. All the ground runs, hot and neutral runs are as short as they can, go to the central point and loop area is reduced. The whole room power references to that panel as the “zero reference” for the room. In fact, the run from the breaker subpanel to the main panel is irrelevant. Because all the audio and power (power is part of the audio circuit!) are locally run.
Think of an airplane. How do they keep all the navigation clean without a ground rod? By using the principles of loop area. Everything is in the skin of the plane and follows the central point approach, to my knowledge.