Looks pretty! Nice idea to adapt your existing sub.
Granite is a nice idea, grogeous to look at, stiff and heavy, but think about how you will attach the driver, and how you will get the granite parts made to the precision required. Can they drill the bolt holes small enough? Laminating MDF + plywood is pretty free of resonances.
The drivers will vibrate a lot and the coax assy will be front heavy, so needs to be bolted or glued down to the subs.
A loose drawer on the bottom for amp will rattle around, so think of a way to isolate it from vibration probably need to bolt it down somehow, or prevent rattling with rubber strips, etc.
Don't let the tweeter be too low. It should be about 40" above the floor. Rebuild the sub boxes if necessary to avoid looking down at the music, that's annoying.
Are you gonna use Danny's SuperV crossover? Make sure the coax baffle diameter and shape are similar to his cabinet's shape.
Good luck! We want pictures!

Rich