Considering adding a 4th subwoofer of the same kind as my other 3. I have 3 in the boot each in an approx 1.8cf chamber tuned to 32hz so I have no remaining boot space. My idea is to make sort of an arm rest for the back seats which will actually be a sub box. My question is when adding the 4th sub if the box size/tuning needs to be the same as the others to sound good with the others (same freq response) or if i can take advantage of having more space available there and make a larger box to sound better (more efficiency/hits lower than other 3). The 4th sub will be on its own amplifer, the other 3 are on a single amp as well.