Excellent question. I am about to build these shortly and have had to make some changes in the drivers due to the high cost of shipping the recommended drivers to Canada. A local company (Q Components in Waterloo ON) has sugested the Eminence 12LFA for the woofer.
The tech there said to run it with out the sounding board.

Are you looking for more bass-hence the tuning board with the suggested woofer?

Ray

As I mentioned I have not assembled mine yet.  
Steve DID  mention that most pro woofers would work well in the box as is. But this leads to some questions for my limited knowledge:
1) Are ultraflex cabinets tuned to a certain frequency like one might have in a ported design AND at the same time a "forgiving" enough design so that one could use drivers with very different T/S parameters and still get decent performance just a different sound?
2) Does the tuning board simply allow one to tune the cabinet to the woofer's Fs?

You are using the Goldwood woofer right? Fs 68
You got deeper bass with the sounding board?
I was thinking of the Eminence Delta pro 12 with Fs 51.  

By using the tuning board, I would assume that one would lose that
+ 6db boost at 80 Hz which would certainly change the sound and punch of the original design?