re: Car Audio - amp outputs polarity?

Are you getting sound out of all the speakers? It is possible your issue is related to the speaker or speaker wire on that channel rather than the amplifier itself. If any are not playing properly, try swapping a working and nonworking speaker connection to see if it follows the speaker or the channel.

And AC is the way to test the speaker output since the output to a speaker will be a sinusoidal wave to replicate what a sound wave looks like. Additionally, the speakers use coils to produce the sound which use AC to generate the field needed. While you could use pulsating DC to do the same thing that is much less efficient and adds complexity to the components as well as additional points of failure.