Hi ImcLoud
When we decided to build the speakers, it was going to be pretty much a floor to ceiling, Upon completion it would be a bit more than 7' No more space for Subwoofers.
Also with the surface area from the woofers, it would provide a good amount of bass. Likely much more than my current 15" Velodyne
Having this setup is optimal and would provide the best performance for a line array. (Floor to ceiling)
I have also intended to use the Funk Audio 18.2 which is phenomenal subwoofer that can keep up with the array, and to boot possibly a pair some point in the future
(Although not within my budget yet)
You can see the chart below... And yes I have listened to these, they are incredibly clean, detailed and sounds naturally fast even better performing than the LMS Ultra. A few guys in the Vancouver GTG this year thought the same. One of them coming from a guy with 4x 18"s (Not Ultras)
You can also get Funk to build one for you the way you want it. Of course, he will give you ideas as well as to what will perform best.
If you are looking for a Nice Subwoofer, You really should talk to Nathan to build you a "Wharfedale sw380" Funk Audio Style.
Here's a link to his 18.0s
Funk Audio - Subwoofer - 18.0 - Funk Audio
Of course you can ask him for a down firing version, don't know if you want to hide that beautiful driver though