You shouldn't use a 14 volt battery in a car designed for a 12 volt. While it should work relatively fine, the wiring and alternator isn't designed to handle the increased voltage required for operation.
A second battery wouldn't help while the car is running. It would let you run the stereo for a longer time while the car is off however. A capacitor may or may not be needed depending upon how much power your amps draw on hard hitting bass notes.