For the DIY route... you can take TC-Sounds out of the equation, as they have raised their prices now to "Out of this world status"
Im glad I pulled the trigger on my Dual 15" TC-2000's...
And that was a good suggestion on the external amps...
You can make 2 boxes, each with a 12 or 15" driver and buy an external amp like suggested, and have awesome bass for a fraction of the cost, but in the looks dept, if thats important, SVS is hard to beat for the money....
The CSS products are excellent drivers, and the kits make it a no brainer for the money.... I have read about lots of builds with CSS drivers... They measure as good if not the same as the medium quality TC-Sounds drivers..
Good Luck with your descision...
Warp