I'll make the suggestions simple:
1. Don't pick an amp until you have a sub
2. don't pick a sub until you know where you plan on mounting it
A small cabin (our cars) coupled with large reflective surfaces (read rear hatch) do not take much speaker to fill the air. Say you have your sub mounted in the trunk facing up. the air will reflect off the rear window forward, almost acting as another form of amplification. a real world example would be to stand to the side of a fan and use a piece of cardboard to reflect air from the fan towards you. similar enough concept.
what kind of music do you want? I built a 2x 10" JLw0 system for a mustang (custom ported box @ 30hz) w/ 55m wts behind it (RF Power) and a 5 1/4 focal sys up front. I could hear the car coming a 1/2 mi away (measured it) with plenty of volume left, but so what? The bass was way overpowering, even at a low level (its what my brother wanted, for people to hear him coming).
figure out what kind of music you want first.
second, to echo figgie, cleanliness is very important in car audio. get a good HU w/ 4v outputs. If you are serious, get a dead head (one w/ no internal amp) Eclipse might still make one, Alpine did a few years ago, Clarion hasn't since 2001 (i have the last model). No internal amp, no heat, cleaner signal.
3. listen. go test out subs. do you want a kevlar cone, wood pulp, alum or poly? they all sound different, what fits your taste? How much space are you willing to give? a quality single 10 in a properly built ported box sounds better than an equivelent (or step up) 12 in a small sealed box IMO. YMMV
4. what is your front stage? $45 2 way kenwoods are not the answer. I say spend the cash for a decent comp set-up with silk tweeters. others may like alum or titianium. what ever fits your taste.
5. now you can figure out an amp. you will know exactly what you need for power at this point. there is no reason to buy an 800w mono class D amp when your entire system only requires 450w to sound balanced. the money you spent on the extra 350w is nothing more than a waste.
just a thought...HTH
