can a different 8" sub fit where the bose sub is and can the bose amp power it

 

yesss.
as long as the bose amp is good.
otherwise you may as well get a box,new sub & new ampp

 

Um....I wouldn't recommend that!! if you plan on replacing the bose sub you should get a seperate amp to power it 1. So you can have control of the sub and 2. Bose impedance is usally very low and you probably wont get the same output on the new sub! And you can fit anything up to a 15" back there with the right amount of work