Dustin,
Maybe you should email or call someone at Adire and see what they have to say. I'm not sure you really need to have 2 drivers to get the performance you are looking for. And the building materials won't cost more than $70 CAN for a single sub, unless you need more than 1 sheet of MDF.
I'm not sure about using an isobaric chamber, but just by looking at the freq response graph in the vented box pdf, the 142.5L box should compete very well with the Paradigm pw-2200... or at least up to 80Hz. Once mine is ready to go, I can actually do comparisons between my Shiva and a friend's...
I started my 142.5L EBS Shiva sub a week ago...check out some of the pics:
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Once it's completely done, I will also do up a frequency response graph and show what one of these puppies can do.