If you're going to install one or more amps with switch-mode power supplies in order to drive a couple of ghetto woofers, you might consider installing a second battery in parallel with the original. Amps generate quick power demand changes, and the alternator/regulator alone can't keep up with the additional load. You should also install a high amperage isolation switch in the hot paralleling jumper. Use cable at least the same size as your present battery cable, and use good, stranded power feed wire, fused, at least the size recommended by the amp manufacturer. Marine stores are a source for all sorts of good wire, connectors, and switches. Remember that paralleled batteries, especially if they're different in rating, will tend to discharge each other over time if left connected with the car "off". Use the isolation switch if you leave the car for more than 3 - 4 days. (Need any vacuum tubes? I have thousands of 'em)
Capacitor banks are great for instantaneous drains, but they have to recharge also, probably from a switching power supply. The second battery supplies the amperage to maintain power supply voltage. Years ago, I ran a PAIR of Hafler DH500 amps in a home stereo system, and even though the Haflers each had 4 huge power supply capacitors I could flicker and dim the room lights because I was overloading the 110V circuit. The solution was a second, dedicated circuit.