alright here's the deal, there is no reason a powered subwoofer can't be fantastic. In home audio, this is all you see. But for car audio, powered systems are generally not of the highest quality. They fill the niche of providing quick and easy "bass" for people who have never heard what real bass is. What a powered subwoofer is, is a subwoofer in a premade enclosure that has an amplifier built into the enclosure. This leaves you with little opportunity modify the system in any way. And also, because of this design, there's no way to test the output of the amp and therefore, no reason for bazooka or whoever else makes them to list its ratings faithfully. Basically, I don't think anyone is truly happy for long with their decision to go that route. I have heard some good things about infinity basslink, so if you really wanted to go the easy way out, I'd recommend that. Unpowered subwoofers require more thought, you will need an enclosure for it and an external amplifier, but in the end you'll be much happier. Hope this helps
I agree with scuba, while powered subs can save a lot of space and thinking, they don't have nearly as many options as if you brought a sub and a seperate amp. It depends on how much bass you like, I mean you could just buy an amp and use that on your rear. I haven't heard a powered sub up close, so I say go somewhere like bestbuy they should have one hooked up and see if it's enough for you.