The #1 thing you can do to increase power output is upgrading the alternator, either by having the stock alternator beefed up or getting an aftermarket one.
The second best thing you can do is to upgrade the stock wiring, particularly these 3: 1) alternator positive to battery positive, 2) battery negative to chassis, and 3) engine ground to chassis. Make them all the same size as the largest power and ground wire your using for your stereo system. Actually, make them all at least 4 guage, you wasting your time making them all 8 guage or samller, make them all 4, 2 or 0 guage.
If you go the alternator upgrade route, doing the wire upgrade is a necessity anyway.
Adding batteries is only useful for running the system with the car off, not helpful for increasing overall power.