true...however if your voltage is over 12 volts you have little to worry about. The chips in car amps are created to operate at low voltages, so it'* not a like a 40 volt chip in your reciever that only gets 30 volts, and still sounds good, but isn't operating at it'* most efficient.
The amperage, or current that your battery can supply over time (not peak....we all know 600 CCA is more than efficient for a heavy hit) is very crucial. And the current/drain curve would neat to see. Of course, I'd suppose the voltage drain curve would be neat to see as well.
I've never messed directly with car audio amp chips, but if they're like high power amp chips used in recievers, they're pretty lenient in the voltage they recieve. I'd guess that a reciever would work fine on 8 volts (assuming voltage protection circuitry doesn't keep the unit from powering up).