It all goes back to the interaction between amps, watts, and volts - and in this case, it's the volts that makes all the difference. A home theater amp operates on the 120V power available at the outlet (at least as far as the US is concerned - Europe does things a little differently). Since watts of power are related to volts X amps, an amp supplied with 120V will draw approximately 1/10th the amperage of an amp operating from 12V.