i don't have rear fill any more but my fronts are full range and the subs are Low Passed at about 55Hz that is with 2 10's in sealed boxes and a 12Db per octave xover //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif not the most conventional xover points but they sound better and are better on the RTA and ears
tuning all revolve's around the res. freq. of your car. if your car resonates at say 50hz but you build a box that the port is 40hz you won't produce as much SPL. my car res. freq. is like 50hz so i built a box for an L5 15" that is tuned to 48hz. find the res. freq. of your car and build your box to those specs.
hate to tell you but you don't tune a sealed box. oh and there is an exact formula for figuring out your res. freq. of your car but i don't remember it i will ask loyd tonight maybe.