| At the moment, I have 2 Alpine 10" S-Series subs hooked up to a 400W Audiobahn Amp. (Regardless of what my sig says; I'm upgrading soon. Each sub is rated at 300W RMS, so I'm going to go with a 600W amp.) I can tell you that the bass on this setup is pretty rediculous. My car rattles at times; the licence plate (need to get some foam padding under there), the rear-view mirror, etc... Soon it'll be pumping out 150% of what it is now.
It all depends on what kind of music you listen to. For rap/r&b/other bass-heavy music, you may want to go with a pair of 12" subs for even more power. But for this car, and especially if you listen to rock of any sort, two 10s would be great. As for brand-names, I personally like Alpine, although I can't afford Alpine amps. Audiobahn is good, and I've heard that MTX puts out good products. I'd stay away from Sony, but that's just from what I've heard, I don't have any personal experience with their amps or subs.
Make sure that you also upgrade your speakers, if you haven't already. ATM I still have my stock 17 year old BMW speakers; I can't wait for my Alpine 6x9s to arrive.
Good luck with your system, i'm sure it'll be "bumpin" soon.
-plusnone
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My 1987 325 is no longer. Rain and an relatively inexperienced driver are not a good combination, to say the least. Luckily no one was seriously injured. | 
