Have you come to a idea of box size on this... Nice build buddy....
I would imagine you could do something around 6^3 or a bit less, very close in design to the size of the Submersive...
I went a bit large on the XLERATORS, and could have gone smaller, but the size of the box was acceptable. I would agree you may be a bit shy on the amp power with about 500-600w per driver at 4ohm...which is just a rebadge of the EP2500, and you can't run the drivers together with it bridged for a 2ohm load either, it just won't take it....
That is the biggest problem with the TC-Drivers, not enough coil / wiring flexibility... If you add another EP400 later down the line, that will be a good inexpensive upgrade in power, or a single Marathon 5050, 1 driver per channel....
What are you going to EQ that with...