EV does not as far as I know publish T/S parameters for their drive elements , so if you wish to truly engineer something you will simply have to measure and model. It may or may not be a good candidate for horn loading.

But that said their subwoofers are aimed at the pro-sound market. I suspect you are going to find the Fs for the 18" woofer to be higher than you might expect. That driver is normally used in a vented enclosure and is intend to have a steep high pass filter applied at 35- 40 Hz to limit low end excursion and output.

Cyclotronguy