i've got top quality competition 5.25's up front, 2 sets of tweeters, stock rear speakers(bout to be upgraded) and a JL 8W6 in my mr2. There's enough room to put up to a 10" behind each seat, however, the speaker box size depends on what size and brand sub you buy. I made my box myself to better suit my needs and the contours of the car. In my opinion, you really only need one 8". Just remember to apply ample power to your speakers. Amplifiers are what makes and breaks you, sound quality wise. Alpine V12's are exceptional. Invest in a decent headunit as well. I'm running alpine head/amps, Kicker comp speakers, and JL sub. Extraordiary, crisp, clear, sound with enough thump to shake the crap out of you. Hoope this helps.