A great separate sub is always better because of placement, extension, and output potential, but I don't see the harm in running an extra to the the powered section of the def techs...
I recommend trying the def techs first, getting an SPL meter and finding out what your in-room extension and output end up as. If it's not low and loud enough, or if you have extreme nulls in the response, get a pair of subs (maybe two rythmik 12 inch servos?) and put them on the other end of the room in 7.4