Arron, If Randy will ever commit, I'll be building one for him! (I'll kill him if he goes with anyone else!) :) But it's not hard to do yourself if you have a table saw, and a jig saw and/or router. You can even do the cuts with a skill saw if you have some sort of straight edge.
Ran, Some amps have built in XO's but also have an LFE input that bypasses the XO altogether. You could (heaven forbid) sacrifice 1" more in height to accomodate the Hypex HS500 from Adire. That is, unless you and your buddies sit around with tape measures in your hands when you get...
Randy, There's the 12" Brahma, which is very similar to the Tumult, but it's designed for car audio. With an LT circuit and the right amount of power it can work very well in the home environment though.
I've got 3 boxes just waiting for the 3 Tumults I ordered. I've never heard an SVS sub though. Maybe someone in the Atlanta area would like to do a comparison once I get my drivers in.