Isn't that the truth Mike...
Craigsub put it out there properly, because JL's accomplishments with the Fathom line are hard to topple for its size and abilities. Even a 15" LMS would have a hard time in an enclosure that small, it will run into some of the same issues as the F113 has with minor compression, but you will need a stupid sized amp to run it and good luck finding that for cheap. Heck just the driver and amp will easily exceed the price of his requirements, so thats out the window.
Its hard to beat what JL does and if you really look hard at the numbers and criteria they are working in, the Fathoms are a pretty good bargain.