I've used quite a few of these drivers, and from the standpoint of someone with plenty of experience with them, I really can't recommend them for subwoofer duty. They'll be alright, you can probably get flat response to 35 Hz, but there are better options IMO.

Adam

You're right... I DID mean to say the Kappa 15LFA was the driver with the nasty peaks in FR.

Good eye!
Colin

That said, I still think that the Kappa 15LFA WILL make a decent subwoofer. Not one with the deepest extension in the world, but a sub that will fit in a cabinet that is acceptable size for a livingroom. Like I said earlier... there WILL ALWAYS be a trade-off between efficiency and deep bass when it comes to subs. If high efficiency AND high SPL is the goal, prepare to have absolutely HUGE cabinets. Most people I know can't tolerate anything bigger than 5 cubic feet for a subwoofer unless its in a dedicated home theater room. If I was building a sub for myself to go with high-efficiency mains, I'd go with an Adire Audio Tempest or dual Shivas with a BIG amp.

FWIW, that is just my opinion.
Colin