to ad a single 8" sub in the trunk of a sedan Vs. replacing the factory speeker with better one?
what speakers are currently in the car? of course it would be worth it to put an 8 in. if you currently have name brand speakers maybe from a premium audio package (like infinity or bose and not manufacture brand ex: ford speakers) then i would go with a small sub.
keep in mind that you wont get the low end that you would get from replacing the speakers. you will get a all round better sound from upgrading speakers and the only way you would get more bass (you may get a little more...) would be to add a 4channel amp to power the new speakers.
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