If you simply want to replace the stock speakers, do just that... replace them. Don't simply use an OEM Bose speaker because the impedance do not match (< 1 ohm typical), so you will risk killing the head unit. It's not worth it.
You will definitely notice a difference in clarity after upgrading your front speakers. As for the rear, you don't really need rear speakers. I don't know if the Base headunit will even drive rear speakers.
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