The Def Tech's will be great to blend into the system, as they go somewhat low, but you are looking for some grunt in the low end...
Email SVS and HSU, they will give you great advice...
I would say either a
PB12-plus or
SB12plus, you'll get a little lower LFE handling with the PB12-Plus.
If you do go smaller, with that room layout, 2 subs would be a good idea..
Placement is harder with 1 in the rectangular rooms. From what I have read..