Hi, im about to order a fi btl in the next few weeks. Im deciding between a btl 15 and a btl 18. I have a hyundai elantra with a 220 amp alternator, 2 batteries and a soundstream rubicon 2500d. I listen to almost all rap and im looking for the loudest system i can build for the lows. I can only fit a 6.7 cube internal volume ported box which is 5 cubes net. I dont know which sub would be louder in this size box. But i really want the 18.