Rule of thumb usually demands stepping down a guage over ~50 feet.
Generally, they'll attempt to balance the load across the two legs.
I did want to comment that you may find that using an existing circuit for one piece of equipment (subs) and then using dedicated lines for the rest is a poor idea for several reasons.
The existing receptacles may have up to 12 lights and receptacles on the same circuit. At each receptacle that the wiring runs through, there is a set of twisted connections inside covered with wire-nuts that may be presenting a small resistance. The more of these connections, the more possibilities of poor, high resistive joints before the circuit reaches the receptacle that you are using. These connections can become highly resistive.
Not to mention the myriad of things like motors, fluorescent lamps, and computers that may also be plugged into this same circuit besides your system.
All this can result in a loss of power and increased noise at the receptacle you're using for your system.
There's also the very real possibility that the overall resistance of the safety ground through the existing circuit will offer a small potential that is different than the dedicated circuits. This small potential difference will cause a ground loop. And what better piece of equipment to amplify a ground loop than a subwoofer.
Although there's nothing magic about dedicated circuits, it ensures a single run of cable from your power panel to a wall receptacle, with no interconnections between and nothing else plugged into the circuit except your system.
When you use two or more dedicated circuits you do have the added possibility of ground loops, but it's far less than between a dedicated and an existing circuit. The best solution for this is to run the dedicated circuits from a common leg together so they are identical lengths and paths traveled. This helps to ensure the potential of the safety grounds are at the same value. The loop is caused by a difference in the safety ground potentials in the system. Even though the safety ground is a cold conductor, it always has a small potential across itself.