If you will push your system to a point where it will draw more current than the alternator will provide, yes your alternator should be upgraded. That amplifier in combination with your vehicles electronics are capable of drawing more current than nearly any stock alternator can provide. A battery will do little to aid the system, if it is an additional battery it can make matters worse because it's another load on an already stressing charging system. The alternator should be the sole source of current any time the vehicle is running, as it provides a bias and the battery(ies) will be in a charging state.