I'd definitly do the 2226 horn - why do a LAB12 horn when you can make one with a woofer you already have! It does 30Hz adequately and is a good woofer so I expect it will be one nice sub. I better add the mouth brace in on mine now :P
I'll make three basshorns, one for my 18LW1400, and one each for my pair of 2225's :P haha. Nawww.... the 2225's stay in the towers.