I think the Bose speakers are 2ohm or less, not 4ohm (which would be pretty standard). Thus, if you drop aftermarket speakers in place of the stock Bose, you'll get less volume, not more. It doesn't matter how sensitive (efficiency is different than sensitivity) they are: they won't play loudly if they have twice the impedance of the stock speakers. There's really not a simple and cheap way to upgrade the Bose system. You might be able to add the
S*bth*mp harness to add a true subwoofer. For anything you can get for $300, that won't be an upgrade (it won't sound good). For $700 you might be able to get something like the Kicker KEY500.1 sub amp and a decent box and sub driver and get louder bass. I'm just assuming you want more bass since you're focused on the rear speakers.