John knows his stuff but I still disagree with the use out of ....my own sense.
What is there to gain using a vented alignment on a set of speakers you're going to cross well above tuning? What is the point? If anything, the design is larger, more complex, has a higher order roll off, has introduced a port resonance and if anything only adds SPL within less than an octave where your current six 18" subwoofer system will best in that area anyway.
I really don't see the point. If you go sealed don't cross. Let them run full range.
You'll get the superior midbass from the TD's and the actual capable deep bass from your subwoofer system which will annihilate your TD's vented anyway. It's a smaller, simpler build which is what you need in your cramped space.
Hey, it's fine with me if you don't want to go this route. It's against the grain of knowledge that everyone follows blindly for decades but everyone does it so it's correct.
Somehow I think you'll enjoy the system either way you go.