I've seen these around a bit, and it seems like more manufacturers are making them... How's the sound quality? Anyone have any experience with them?

http://cgi.ebay.com/06-PIONEER-TS-SW...QQcmdZViewItem

http://cgi.ebay.com/KOLE-MFW-12-Subw...QQcmdZViewItem

http://cgi.ebay.com/EARTHQUAKE-SWS-1...QQcmdZViewItem

 

the pioneer and the earthquake are both garbage. ive only seen one good sub come from pioneer in the recent years.
the kole doesnt look too bad but it still requires 1.0 cubic foot of air space and it 4 inches deep.

look at these here. mounting depth is 5 inches and it requires only .5 cubic feet. and i know that these sound good
http://www.soundstream.com/subwoofers-picasso.htm
overall i think these will work better for your application

 

If you want HIGH QUALITY shallow mount subwoofers. Check out the dynaudio mw-190. The best sq you ll ever hear next to an aura ns and a velodyne.
bp