When I was into bass competitions, I had custom 12" boxes made for behind the seats. It was loud and very uncomfortable. After that I went 10"s behind the seats, that was also pretty annoying. Now I have components front and rear with no subs and it sounds great.
Ya.... I agree with ryde. I have a 6x9 behind each seat, and speakers in my doors... and it is more than enough. If you do put a sub.... don't do any more than 1 - 10"... that should give you more than enough bass.
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.
i have 2 5.25" in the door, tweeters in the mirrior and 4" tweeters in the sides behind the seat, and 2 10s behind the seat in a box that was out of an s-10. it fit perfectly. I only had a little mtx 250 watt amp, and i was really suprised on how hard they would hit. All the speakers were sony, the subs were mtx. The cab in the mr2 acts like a big sub box.