Must be musical, fast, and dynamic to keep up and give me the visceral kick drums I'm looking for. I listen mostly to Jaz / Blues / Classic Rock.Are your two B-15s in the corners behind the La Scala's? Why not fill the other corners with the B&Ws slaved from the B-15s?
Two B-15s should be more than enough subwoofer. I don't buy into the speed and musical aspect. In my experience speed is usually associated with sub placement.
Along with location, poor optimization and EQ control hinders integration or musicality. Kick drum timing is often an issue with polarity which can vary from track to track on the recording recording itself.
Matching with a horn sub when your already tight on space? Not exactly sure how but Magico seems to do well with a sealed box with their Ultimate One's.
While Revel's B-15 Optimization procedure is quite dated it offers much more control than many of the suggestions here. I wouldn't use Revel's suggestions of Y connectors and high pass filter. Are you using their LFO devise and disc or a 1/3 octave RTA?
Good luck with it.