ok so the box is around 5 cubes and your only have 31 square inches of port. that gives about 6 square inches per cubic foot, you need at least 12 square inches per cubic foot. that will make the biggest difference for you at the moment. thats just rough math but it clearly points out your weak point.
since the seats do not fold down i would, if you havnt already, take the speakers out of the rear deck. they wont do you much good for music anyways and it will let a lot more bass into the cabin.