I'd also agree with all of above. A cap can work well if your existing charging system is right on the verge of being enough. If you put a multimeter on the amp's terminals and it's only dropping .25 - .5 volts at idle when it's jamming a cap might be a decent solution. Any more than that the cap is just not going to deliver enough to make a real difference.
The reason a cap can sometimes help in that specific situation is it's ability to deliver the electricity faster than the battery. The overall resistance of a cap is much lower than a battery plus it's located very close to the amplifier. So in that instant that your system demands more power the cap can fill the void between the battery and the amplifier, until the battery's power gets out of the battery and through the wiring to the amp. The problem is if your charging system is underpowered for the application you will frequently completely drain the cap which then becomes just another load for your alternator to charge.