To the general question (horns with subs) rather than the specific K-horn scenario, you could check out Avantgarde speakers which use that approach. I've heard them many times and have never noticed a problem with low end integration.
FWIW
FWIW
Horn Speakers and Sub Woofers
How difficult is it to mate conventional subwoofers with horn speakers, in particular Klipschorns. I'm moving my 1987 Khorns to the large living room for a 'home theater' setup. (I have two good corners for the Khorns). I am thinking of adding one or two medium sized Rel subs to the setup to use for movies. Have you tried subs with Horns or corner horns?