sony xm 2200gtx amp thoughts on subs

I've got the sony XM-2200GTX 2 channel thats supposed to be rated at 250 watts RMS x 2 at 2 ohms... so.. The subs I have right now are two 12" xplod's I bought on sale 6 years ago for 79$ a piece.. I just spent 1200$ on new component speakers, 6x9s, an amp for the speakers and a h/u.. but decided (which now I regret) that I was going to save a bit of money and use the subs and 2-Channel (which still works and puts out a decent amount of power without going way over budget) I had been pounding the hell out of for the last 6 years and really I do like them, for the price, they're amazing. but now that I have stepped up the rest of the system I've decided I'm going to invest in some new subwoofers and build a box custom to the specs of the manufacturer. I've been looking around online and have come across Infinity Kappa 122.7w (are these old subs?), Rockford Fosgate P3D212, and Alpine's SWS-1223D. So the price cap is around $200ish (Canadan dollars lol) I know people who have purchased from the canadian crutchfield website with good success so this is where I'm getting all my pricings.

I know that seems like alot of info to take in but to anyone who is willing to suggest anything I'd greatly appreciate it, this forum has already helped me once (in another section) so I figured I'd give it a shot over here..

Thanks a bunch,

I'm still new at the whole car audio thing really, I know I love music, and I love driving.. so loud music in my cars always made sense to me.. so I usually went to the local Future Shop and bought something that a couple of the guys there would recommend.. but who knows really if what they suggest is what's best for me, or whats best for their commission. But here online you guys aren't getting paid either way whether I take the advice or not so to me it's more honest and true advice. With that said, I'm asking for people who have experience with all ranges of subwoofer sizes in all ranges of vehicle sizes.. I mean.. what would a 15" quality sub (Kicker Comp VR 07CVR154 or similar) sound like in comparison (SPL and SQ) to two quality 12" subs (Rockford P3?) compared to a single higher end type 12" (Alpine SWR-1242D)    any of these combo's in the back of a 99' Buick Century so it's got a fair bit of space to fill.. I've had a single 10" sub in an Intrepid before and that was horrible so I'm justifyably (to me anyways) skeptical when thinking of replacing 2 speakers with one.. I know alot of this comes down to experience and it'll take a long time and alot of money for me to really understand what to look for when purchasing new equipment.. that's why I signed up here so I can pick the brains of some of you Car Audio Nerds (I mean that in the nicest way possible, honestly) and make my Buick stand out from the other Grandpamobiles.

Thanks!