Looks like we were writing our posts at the same time. I think I've partially answered your questions already, but want to add a little summary.

The transformer does not convert from AC to DC, but instead changes the voltage. A transformer of some sort is necessary in order to get the voltages involved down to a level that speakers can cope with. I believe that toroidal transformers are most often used in home theater amps because they are the most effective way to pack a transformer large enough to handle the required current - although most receivers elect to use a "square" transformer (technical term there...) that looks like a scaled down version of what you might see sitting behind a building to save space inside the cabinet.

All speaker amps are working with DC power at some voltage when it comes down to actually driving the speaker, so the amplification section itself cannot directly accept AC power. The only way to change the amp's power input from 12VDC to 120VAC is to build the sort of power supply (transformer / rectifier / capacitors) that home theater amps use - thereby producing DC power that the car amp's existing power input can accept.

Sorry, no clue.