I'd definitely agree with Darkstar. I built a WO before I knew better - the original version, not the newer one. It's not a horn. It's a funky bandpass box. Sensitivity was not increased any more than you would expect from having two drivers running together (at least with the drivers I used). It dropped like a rock below 40hz, if I remember correctly. When pointed into a corner it got down a little lower but needed a healthy dose of parametric eq to sound decent.In regards to using different drivers in the same horn, it's a total crap shoot unless you're the one designing it and know what you're doing. In general, you should design a horn for a specific driver.
If you really want a horn subwoofer, you need something about 3x - 4x the size of the WO (about 30 cubic feet of horn should do you fine indoors).
John