I have four. Each powered with a MRD-M1005 amp. The sound quality is great, and the SPL, well, I havn't measured it yet.
I also have two 12" SW9122 Eclipse subs in my Honda Accord. I can turn each Alpine off seperatly, and each is in a different enclosure. When I compared two Alpine's to the two Eclipse, they're about the same in volume, but Alpine has better sound quality. Just thought I'd compare it to something for you.
The price difference is about half the price for an Alpine. I'm VERY happy with my Alpine's, and that's the route I would go if I were you. (hope this all helped I know the way I stated this sounds like I'm retarded, but I havn't slept in three days :P )