spyd3r = right.
And regarding the good amp against bad amp, the difference only occurs once the setup is done improperly. My Rival is a perfect example of this. His 2 amps? A Pyramid Gold Competition 600 watt amp, and a Viper 400.2. I've heard both attached to his subs, and mine in both his and my vehicles. The difference? Well, all I can tell you, since watts is watts, those extra 200 watts in the Viper must be a gift from God. It's the only way to explain the richer, deeper, louder, cleaner bass coming out of the Viper. My PA amp can hardly keep up, and I consider that a pretty nice amp.
As for class A amps... meh, I'd much rather take a class G, I'm almost certain at the same power output, distortion would be about the same. Plus there's no pesky water cooling system. Although, you could just use the car's frame as a heatsink....