it would be great if you compared your f206 vs 226be. I wouldnt buy a 3rd sub until you measured all the various possible positions in your space.
I would have to disconnect my f206's and bring them to the front of the room to compare apples to apples. Not that big of a deal, but not really worth the hassle.
I'm fairly confident that I'll be okay with the two subs after using the miniDSP and REW... We shall soon see!