i don't wanna be mean but please stop looking at sony explode subs. there're not worth your time unless there free and even then thats pushin it...
whats your budget? what type of sub are u looking for (e.g. SQ, SQL, SPL) if your looking for a sub by yourself look for one thats DVC (Dual Voice Coil) b/c you can wire it in parallel and buy a cheaper amp and power a dual 2 ohm sub at 1 ohm. some simple physics would help a little when your talking about series and parallel circuits. look them up.
also try to understand such equations as P=V^2*R. this means that if you lower your resistance you will get more power assuming your voltage is constant. its hard for me to explain so just look it up on google or something. get back at me if you need help.