I don't understand why you are putting a single 8" subwoofer with two 12" subs. It will hardly be noticeable.
I would much rather put a 8" inch midbass. That would be much more beneficial than another subwoofer.
That box is sweet looking. Just wonder how your going to keep those amps cool?
PS: What kind of tools did you use to get the wood so smooth?! and rounded like that.