The crossover network in the speaker is focusing on the transitions between drivers (tweet/mid/woof), not on the roll-off at the bottom end. The bottom end is likely determined entirely by the interaction of driver and enclosure, and with the enclosure serving as a constant I doubt that treating the driver as a variable (changing it out for a different driver) will buy you more than a few Hz.

Purpose-built powered subs would likely be the best solution to achieving really strong deep bass in your room. The idea of going with two subs in such a large room is also probably going to be for the best. Depending on what music you listen to, the Horns may very well be doing all you need - I know that I felt much the same with my Paradigm Studio/60's when I ran them without a sub. Movies definitely can benefit from a sub, though, and for some music the sub can also help.