If they already shake the windows, why even buy another sub? Are you trying to also shake the floor and ceiling?
A long time ago, I used to own the DT BP7000SC for fronts and BP7001SC for surrounds in my 15'x17'x15' (3,800 Cubic FT) room. They rattled just about every wall, window, and door in my 3,000SF house. Even so I added the DT Trinity sub. I'm sure it produced more bass. But since all the walls, doors, and windows were already significantly rattled, what's a little more rattling and shaking?
I think in this hobby, we tend to over-do and over-think things.