The subpanel under your system is preferable in minimizing potential ground loop issues. Are you planning on using both legs of the 240 (120vac-120vac) from the subpanel?, perhaps one leg for your AV equipment and the other leg for other uses to keep external noise out of your audio. If you're only going to use the one leg, just for powering your system, if within code, you could save by only running 1 of the 2 120vac legs and be sure to have the subpanel labeled as such so the next electrician dealing with it isn't totally flummoxed. 6 gauge solid core is indeed expensive. Lastly, I run my system off a single 20 amp circuit. No subpanels required, minimal potential for loops. But I also use multiple filter power conditioning that isolates components from each other.
I'm looking forward to your report(s) on how this investment in resources effect your system.
Regards,
Paul