My guess as to why khorns sound better in larger rooms is that they have a chance to really open up. Being 100% horn loaded, and being in a 1/8 space (corner), they are able to couple to and control a huge amount of air. Really gives them a "live", room filling sound. Even though they are not at there best in smaller rooms, they are still pretty impressive.
Not sure if ceiling height or simply room volume is more important. You might want to pay attention to the first reflection points on your low ceiling and treat these areas somehow. Probably best to use diffusion if possible, absorbtion if necessary.
Again, good luck and welcome to the forum!