While it's true that subwoofers should be judged as a whole, the driver is the heart of the sub and it determines the performance more than anything else. If you are going to build a sub, you start by selecting a driver that will meet your criteria and go from there.
There was a $15k sub called the Ulric from a company called Lycan which used the LMS Ultra 5400, but they don't seem to exist anymore.