Car speakers are designed for....a car. They may sound weak in a big room like a garage no matter what amp you use. You might try running this by the guys in the car forum here, maybe the speaker forum, too.
Still need to know what your source for music will be. A power amp does not have a pre-amp built in, which you'll need for your sources to feed a power amp (like cd players, mp3 players, etc). An integrated amp has both a pre-amp and an amp; an avr is a combo of a tuner/pre-amp/amp as well as providing mucho connectivity options (including video which you may or may not need but still the best buys are generally avrs). Most AVRs don't have the kind of power passive subs require, though.
"I realize that somebody playing free music isn't as commercial as a hamburger stand. But is it because you can eat a hamburger and hold it in your hand and you can't do that with music? Is it too free to control?" - Don Van Vliet (aka Captain Beefheart) discussing commercial success in the music biz