Well, I built an LLT using the 15" TC-2000 and have been very pleased with it so far.
Yeah, a 12-13ft^3 MDF enclosure would be pretty **** heavy, which is one of the main reasons I chose to go the sonotube route.
Most folks make their own jig to cut the big diameters, but there is a jasper jig available that is capapble of much larger diameters.
http://www.rockler.com/product.cfm?page=5960