$600 to spend on new car audio system.?

In the audio world if you want good sound then that's not allot of money to work with but if you shop in the right spots you could make it work. for your stereo (deck) i suggest pioneer or alpine they are great and you can get them with ipod adapters. For a subwoofer I suggest JL Audio and if your looking for some clean sound but don't need a whole lot of thump you could look at just a 10" subwoofer or if you want allot of thump (which you can still get clean sound out of) you can get a 12" or 13" sub. As for a box you have 2 main choices a vented or sealed box. The vented box allows more airflow so you will get louder sounds but less cleanliness and a sealed box does the opposite you will won't get as much volume but it will sound cleaner. I suggest you look on http://www.onlinecarstereo.com/

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go there, join there forums, ask questions, be respectful, learn.

Best thing you can possibly do.