I had a 1.5 farad cap in the line with my system to "stiffen" it up, but multiple batteries is definitely the way to go.
The tech seminars I attended when I did car audio showed the difference between caps and batteries by connecting a light bulb, which would take a few seconds to go out with the cap, but would stay lit indefinitely with the battery. Sure, a cap can recharge but....
They also showed the voltage on a scope with a cap vs a battery- it was more constant under load with the battery.