Confused in what to look for when buying a car amp... Watts, Db, THD?

The amp is dependent on your subs. I will use a simple example that can easily be translated. First off, screw the Db thing its pretty much pointless for this matter. the THD is important to determine sound quality. if you have like 8" or 10" subs you want to keep that number low as possible. but when you get to like 12 and 15 inch subs it loses its significance. Now for the example, Lets say you have two kicker 12 cvrs? each speaker lets say has a "RMS" (this is the key term) of 400 watts. If you have two of them you need a total of 800 watts RMS. Now you want to actually run your subs on that 800 constant. So if you buy a kicker amp for example. You will get a certificate that comes with it that signifies that on that amp it is pushing "X" amount of watts. Since your subs require 800, you want an 800 watt amp. But be forewarned, cheap brands like sony and pioneer arent going to give you the power they say they will. but better brands will generally have the right wattage rating. Always always... make sure that your connections are good. You want optimal output in your speakers? have your RMS of your speakers matching your amps output, and that your using a good sized wire at the right thickness. if the wire is too thin you arent going to get the energy flowing through it properly and always have a good secure connection from the speakers to the amp. Also if you want good sound make sure you are using the proper box. the box determines sound quality as much as the subs and amp do. ported or non ported. slanted or square. plexi or no plexi. The box is determined by the size and shape of your trunk. if you have a big trunk. i would suggest ported. solid 3/4 inch MDF wood. for a small tight trunk. regular slanted box shaped box. no ports. but also this is determined on a good part of what kind of subs they are and the amount of air they move. this is easily determined by looking at the size of the surrounding rubber. is it a thin surround? or is there a big wide rubber surround? common sense says that if it has a thick surround then it has a wider range moving in and out meaning it pushes more air and therefore a ported box would help it move a great deal more. ah. i could go on and on. i dont want to overstimulate your learning mind. use these instructions and tips and to you my upcoming boomer... Good luck.