I have two Outlaw x-13s in a room, about 2000'3. Please tell us your definition of overkill.
One of those is rated for a 5000'3 room!
OK... square rooms are rough. You will likely have bad standing waves that will kill bass frequencies here and there around your room.
Sub Crawl, though crude, is your best way to dial in for a single seat. Next best thing is going to save and buy a second sub!
Without knowing your whole scene, I will suggest that you keep your mind open to subwoofer placement options as you do the crawl... don't let unconventional placement scare you. That said, you will likely be best served having the sub asymmetrically placed so that reflections from the other three walls aren't getting back to the sub at the same time.
Hope this helps!