Not only is it easier to buy a powered subwoofer, it'll sound better as well. As good as some car audio subs are, they are nowhere near as good as those designed specifically for home theater/audio.
cabinet design has alot to do with that, i've built boxes for car sub use in a home and they sounded great. they are after all, just speakers and don't know wheter they are in a car or in a house. it's the downfall of many a speaker, not putting it in a proper enclosure.
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It's "Dr. _paralyzed_" actually. Not like with a PhD, but Doctor like in Dr. Pepper.