look, if you want to put something for rear fill, use round drivers. get urself some shallow depth 8s. like the new jl w1 series speakers. theyll give u a great boost in midbass and some bass. dont expect it to sound like theres a massive array of subs in the car, but you will notice a difference. also try to make enclosures for them. use fiberglass, try to fit in a basketball's amount of air in therm. shape doesnt really matter, theres enough room in the back there for it. you also want to make sure that ur crossovers are set properly for those 8s, leave it all on midbass, cut out all the other frq's. there really is no need for rear fill, ppl just do it to make it sound like theres more sound coming out if they lack the power. if you have a good set of components upfront that are properly done, aimed and time corrected, there will be no need for rear fill if you sit those on a good solid amp. in my car it seems like the sound comes from all directions, and the voice comes from the center. if u want i can scan my sheet from this past sunday, the 7th. i got 2nd place for SQ. i can post photos of my audio setup if anyone calls bullshit. if you run your speakers from just the deck, its ok to run rear fill, the deck doesnt provide all that much power to start with and a typical deck only puts out 1.5-2 volts RMS. the one i use puts out 8v rms so i dont have a problem. if you dont want to splurge a lot of money on a high end deck, spend about 200 bucks on a matrix 6 channel line driver, they have a built in eq and prolly a crossover that has a much better range that the deck u use. itll up the voltage on the signals to ur speakers and sub/subs at the same time if u have a sub/subs. itll give you a much cleaner more powerfull signal with a better signal/noise ratio. youll notice a huge improvement in sound quality if you do that. having a nice solid amp also wont hurt you. just stay away from stuff like pyle or lanzar or boss. they write numbers that you will never get out of their amps. with a speaker amp $1.15 per watt is a good range. go with a proven name in amps. im using an MB Quart amp, which sounds absolutely amazing, i would definitely recomend it. the one drawback is the price. not everyone will want to pay that much for just a 4 channel amp unless they are planning to compete. also keep in mind that if you have a weak headunit to start with, theres no point in buying an expensive amp and speakers, itll still sound bad because the signal comin out the headunit will be crappy to start with so nothing will come of that.

I just wish my car was as fast as it was loud. people tell me i should use my sub to push it, it might go faster.