Subwoofer???

Hello Guys,

I have a Kicker ZX750.1 amp (http://www******er.com/ZX_Amplifiers), with 750 Watt Mono at 2 Ohms (not sure if it is RMS or PMPO), hooked up with one Sony Explode ( I think 380 watt RMS/ 1200W PMPO), in a small tight Q-Logic box. I literally have to keep the gain on the amp way less than half, otherwise it distorts the sound. A number of people have been telling me that the sub woofer is the week link, and I will gain a lot just by changing the sub. I do not want to put two subs, or big box in (kind of like trunk space ), or spend a lot of money on it. Below are teh questions I have.

1. Would it really make a huge difference just by changing the sub?
2. If yes, what power sub should I get (RMS/ PMPO)?? Most of the subs are 4 ohm, however; the amp says 2 ohm.. does it make difference? To be more specific, any preferred brand/ model? I looked here http://www.bestcovery.com/best-car-subwoofer-overall, and the best subs range from 80 bucks to 900 bucks.
3. I need a 12 inch sub that works with this box (that can fit in that box, and also the box can withstand it).

Thanks again for the help and support .