The Kappa Perfect slot port build will work fine as a pair, with one Ep2500 driving both cabinets.
You can use sealed as a smaller option to the Kappa Perfect build; the Perfect build will end up a little over 5 cubic feet total of gross volume. But going sealed, you have to use a driver with long linear stroke, which means a higher cost, and higher power requirement, such as needing an Ep2500 per driver.
The Maelstrom-X 18" is an option, if you can manage to get it(it seems to be on backorder most of the time). But you won't be able to use a cabinet much smaller than the Kappa Perfect build would end up. The Sound Splinter 18" is another superb option, or the 15" Sound Splinter, with a bit less output(less surface area, of course), to save some real space. You can use the 15" in a 2.5-3 foot sealed cabinet, and get high, very clean output, if you use a Behringer Ep2500 amplifier and a DCX2496 as the response correction tool(using a sealed undersized cabinet requires electronic compensation, and only works well with extremely linear drivers like the Sound Splinter and Maelstrom X and other limited number of long stroke linear drivers; but this is a perfectly acceptable option and is a method used in such popular high end subs like the JL Audio Fatham and Gotham subwoofer systems).
As long as you can get more than one subwoofer distributed in the room, you will have better integration. Ideally they will be the same distance co-located from each main, of course. If you can cross at about 55Hz or lower, using a 4th order crossover, one subwoofer will integrate nearly perfectly for music. But when crossing at higher frequencies, you usually need multiple subwoofers to integrate with the mains for music perfectly. It's also not strictly about localization. It's about proper response summation with each main speaker simultaneously across the transition/crossover band.
-Chris
