Replay, I can't honestly say the Kappa 15LF would be good for subwoofer duty. The Q is fine, but the fs of 39 Hz makes it a difficult task. I have run mine in a variety of enclosures, the best being a 5 cuft one tuned to 40 Hz. The f3 was 37 Hz I believe. Good for PA use, but insufficient as a true subwoofer.The Kappa 18A will operate in even a 5 cuft ported box. You could also run it sealed. However, we're talking limited extension.
Mine run in 16 cuft boxes tuned to something I do not remember. The f3 is about 25 Hz, though. You can definitely tell.
You could probably get similar results in a 12 cuft ported box. I'll run some simulations for you if you want.
Adam