Back in the day I bought 2 12" Quantum Cozmiks brand new... They did get loud, But they did not get low! I had them on a quantum 1200d (that got hot enough to fuckn grill on) I doubt the amp was doin rated and the subs barely wantd to take what it was given out. But heres a couple links to some good deals on them if you still wanna go that route!
http://cgi.ebay.com/QUANTUM-AUDIO-TWO-15-2000W-DVC-2-OHM-SUB-SUBWOOFERS_W0QQitemZ310203523800QQcmdZViewItemQQptZCar_Subwoofers_Enclosures?hash=item48399222d8
http://cgi.ebay.com/2-QUANTUM-15-6000-WATT-SPL-CAR-SUBWOOFERS-3000w-RMS_W0QQitemZ390134944556QQcmdZViewItemQQptZCar_Subwoofers_Enclosures?hash=item5ad5dad32c