Seems to me the answer is simply to use an amp for the Khorns that has an input level control to attenuate its gain to the desired level to match the gain characteristics of your sub. Then use a preamp with sufficient output voltage to drive the system as needed. Of course you also need s sub with a high max output to keep up with the Khorns.
The response for REL regarding subwoofers
The response from REL regarding adding a REL Subwoofer for Cornerhorns.
"Unfortunately a speaker with that kind of sensitivity is going to be near impossible for any subwoofer to match with. The 2 volumes will never be able to match well. "
Is that true? It doesn't sound true to me. The SPL to power will be steeper than a less efficient speaker but it should jut be a matter of adjusting a powered subs gain curve to match that of the high efficiency speaker output or just using a more sensitive driver in the sub.
To bad I really wanted to buy a REL sub but I guess I'm not understanding how speakers work.