EDIT: My numbers may be off but the theory still applies, sorry :/
This might help too:
Grand Prixs Of North America • View topic - Capacitor Info
Think of a capacitor as a battery that can maintain a very high voltage but doesn't hold a lot of power so it maintains the ability to discharge VERY quickly.
A battery is charging ~12.6V. If you're charging system drops below 12.6V then your battery is no longer charging and you begin to draw current from the battery. A 1 farad capacitor only maintains enough juice for about 1/10 a second in a 1000watt system. Once the capacitor is drained it becomes a secondary load on the charging system. You're cap will not discharge until you drop below the charging state for the battery. However, it will wait until the battery returns to a charging state before becoming an additional load on the charging system.
So in that instance, why use a capacitor?
In addition, a well designed amplifier already has the capacitance necessary to maintain it's output built into it's power supply section.
Although the voltage is low on this...here's a photo of a 1 farad capacitor...
1 Farad 5.5 Volt BackUp Capacitor
Feel like you were ripped off yet? I will admit, this one is only good for 5.5V. A higher voltage would increase the size but you could always wire four of them for 22V and 4F.
Here's another...
CPAG1.0F - 1.0F 2.5V Aerogel Super Capacitor Products Solarbotics
So after it's all said and done, you either need a higher output alternator or a better amp
