If caps are worthless, why are they used in every amplifier's power supply? Because they lessen the fluctuation of voltage, hence the name stiffening capacitor. They store current that is already on hand and deliver it instantly. Now a battery creates an additional supply of current, making the system capable of twice the current draw (assuming you have two identical batteries) Making it a great solution for high powered systems and since batteris install just like a cap, they can easily be added in place or in addition to.
I am not saying they are the best solution, but they are not as worthless as they are made out to be. To say listening to rap or R&B will make the cap ineffective is not a true statement. Do you think it takes a cap a full second to recharge, electricy moves pretty quick (speed of light er some thing...lol) So any break in beat, the cap has recharged. Sure you can have 6 sub amps and one cap, that would be worthless... I am talking average systems, one or two amps and a 1 or 2 farad cap.
Now test tones/sweeps will render a cap helpless. I would say that a cap will improve effeciency of a power supply and in turn improve SQ. Most rap tunes are not used for SQ judging so I guess thats another strike on the cap.... //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif