Is that a prefab box. Those ports seem way too small. Why dont you just get a good pair of 12's and a second amp. Then you would be alot louder. two 12's with a custom box and alot of portspace is what I would do. With your 15's your not givving the woffers room to reflect the bass back into the cab.especially if you increase the box volume. Unless you flip the box around and seal off the trunk completely. At least that what I would do.