08 Altima Coupe Bose modification questions

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I have an 08 coupe with the "premium" or whatever Bose audio. I've been riding with it for three years now and it's time for more bass. I am new to car audio and have tons of questions, but let's start with this: Is it true that with the Bose system I need to upgrade all the speakers and can't simply add a sub, amp, and head unit?

A friend of mine just told me that Nissan uses small batteries and alternators now and that I'll definitely need a battery. Any input on that?