You could get some better drivers then the Kappa IMO
AE speakers has some fantastic drivers The Lambda TD15H, 15M, 15S and 15X give you a pretty wide range of products for different applications. They also have 12" drivers in all those same configurations.
http://www.aespeakers.com/drivers.php?driver_id=10
AE Speakers is just north of me in Green Bay, WI and John Janowitz (owner) came down for my Sub GTG and brought a 15" driver with a 24" box and a couple of 18" passives with him, he/we assembled it in front of everyone, so we got some hands on with the drivers and passives, they were very very nice drivers, and that sub which sat in my house for over a month sounded spectacular and had output that downright SCARY !!!! It had as much output or more then the 18" LMS sealed. The build quality is much better then the Kappa's stamped baskets, while still having very comparable pricing for what you are getting.
AE speakers is producing the drivers for Mark Seatons products, as well as some new drivers for Salk Sound, so they obviously pass the test of some very scrutinizing builders here.... His 12" drivers and passives are the ones in the new SALK HT4, Salks cost no object speaker !!!
http://www.aespeakers.com/shop/catalog/
Get a 12" driver with 2 - 15" passives and that will be a freakin impressive sub, which requires very little power !!!
I will vouch 100% of the SQ from that sub...