Attention all readers: Rear speakers are no good for bass! Thats what a sub is for. After sitting in my brothers car, I really dont even understand why people would argue so much about why rears are "necessary." Trust me, they are not. Even a 10" sub will do, and beat a rear speaker setup any day. And make sure the mids up front are enclosed so that you get better low end frequency (the 80-100 range where the sub starts to drop off). Then just set your sub to cut off between 80-120hz, and the fronts to pick up around 10 or 20hz lower (as stated by nubz69), and you will be happy like this
The only reason I will have them are for rear channel in movies, and split channel mode (like music in front, video game in back).
And someone said that with rear speakers, you cant hear where the music is coming from, but rather that it is just there inside the car. Guess what, its the same way in my brothers car! Just save that 100 you were going to put on rear speakers, and either get something like Boston Accoustic stuff for front channel, or a sub, or something.
And I dont believe in needing multiple sets of components up front. Just enclose them, and for average listening (like up to 120 db or so), you should never have a problem. True, subs can overpower the mids, but that just means you either have the volume up too loud or the subs turned up too loud.
Just my opinion, but spend the money on whatever you want.
Edit: Also, for bass, I would really reccommend 2 12" subwoofers, or one quality 10-15" subwoofer. I am just doing one quality 12" woofer, and look forward to getting it installed. Itll fill up the car nice and good .