Aftermarket sub in stock jbl sub box; access cab

Would it be possible to use the stock JBL stock box, install an aftermarket sub in it, and secure an amp under the seat so I don't loose any room in my truck? I know it would not sound extremely loud, but for loudness I have my other car that is pretty heavily equipped on audio. I would just like to have some more power so I could actually enjoy my music.

Also, I would not want to remove the stock sub and have those 2 openings in the back of my cab. Those are the 2 reasons why I think keeping the stock sub enclosure is the best for me; space and the 2 holes ... and no need for a custom box.

Thank you

 

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