Caps are only good if your vehicle charging system is able to keep up...if it's already starving for powe caps are going to help very little or not at all. You can google search FAQ about caps and you will come up with some good info.

The purpose of a car audio capacitor is to deliver large amounts of current very quickly (faster than a battery is capable of). Which is why they are used to make the car go Booooooooom not Boom. If your car is starving for power the caps won't have a chance to properly charge back up under high volumes.

A second battery will increase the load on the alternator, not decrease it. With the vehicle running the second battery becomes another load for the alternator to charge. Second batteries are only good for engine off listening time.

Get a better alternator.