I'm in the same situation, looking for a good 15" around that price. So far the RD Alpha catches my interest for the price and the reviews they're getting..
if your going for straight out SPL to "tear your car apart"... with $500 or so, i would upgrade my amp and then buy a sub. trade or sell your amp for a 2500-3000w amp and then spend ~$300 on a beastly sub. no need to spend $500 on a SPL oriented sub and only run 1500w to it
edit::: also, check with Korn4Life, he is selling my old 15" Audioque HD3 that WILL tear your car apart on 2500rms. its under $200 also!