The Elemental Designs 7Kv.2 has outstanding SQ and surprising output for a 6.5" driver... I had 2 in a 30Hz ported enclosure with 600+w going to them and they handled it well....... I think they're only $45/ea at http://www.edesignaudio.com with the forum discount..
I know someone who ran 4 of the eD subs last year the eu-700, which is now the 7kv.2's. These are surprising little subs. Hit a 145 at IASCA finals 2006 with four in a box from the trunk of a 4 door! Very impressive little subs. I think he was giving them over 500 Wrms each but not to sure and they are not that expensive and have really nice SQ properties.