The Defcon's don't use subwoofers. If you put a Sub in the defcon, you won't get the desired results.
The Defcon's actually use Mid-bass drivers.
Common Subwoofers have a frequency response of 25-200hz
Common Mid-bass drivers = 65-5000hz
Completely different speakers.
Try calling Skylon and see what driver they used for it. If I recall correctly Skylon was using off the shelf components for their Defcon series. You maybe able to pick up the driver from Parts Express or the like.
Here's a link to Part's Express's 8" mid-bass drivers. http://www.partsexpress.com/webpage.cfm?webpage_id=3&CATID=49&ObjectGroup_ID=5 79
I'd probably go with a Eminence Beta-8a if Skylon is of no help.