i forget the volume of the stock enclosure, but you would want it close to what you plan on throwing in there needs. the stock sub has a mounting bolt through the back of the enclosure, threaded into the speaker. which means you would have to plug that hole somehow before you throw another sub in there. just throwin that out there.
i have the stock enclosure, sub, amp, and carpet cover i am going to post in the BST section soon. gotta take some pics. i had the stock sub connected to my JL amp and it sounded decent. nothing like my 10W6V2 though!