There's nothing to be concerned about with HT, all you have is processed fake sound effects.
A horn stops acting like a horn where the size of the mouth of the horn is 1/4 wavelength. That's why the Klipschorn is in the room corner. It uses your room corner as part of the horn. What I don't know is given the flare rate of an exponential curve, just how big a mouth that effectively is.
One of the other posters here suggested the 7 foot Klipsch theatre sub-woofer. A 7 foot wide mouth is a quarter wavelength for a 35 hz tone, so it only acts as a horn down to 35 hz.
I suggest you just be happy with your K-horns as is and forget the subs.
I have never had K-horns. Always been a big Klipsch fan though. I set up a home theater system with Cornwals as the front mains, Heresy for center, Forte's for the surrounds and some cheap plastic Klipsch for the rears. I used a Def Tech 15" powered sub and it was the best home theater system I have ever heard. Period.
Moved twice since then, all I have left are the 1989 vintage Forte's. Good luck, nothing does movies like super efficient horn loaded speakers.