What is the largest, best reasonably priced sub i can put in place of my old Bose sub in the hatch?

Im looking for a clean bass... The bass on the stock bose system is actually fine with me but im changing the head unit and people recommend changing the bose speakers out too. Isnt the rear sub a 6 1/2 sub???

Speakers will most likely be Polk. 5 1/4 and 6 1/2's
Head unit will be a alpine 9815