Tom Danley got my attention when he said the BMS 4550 works better with conical horns than others he has tried. The stock QSC horn has a pretty sharp initial angle so that might apply.
Aside 1: I'd still be tempted to glue 1/4 of wood on the QSC and go to work with some sandpaper to shape the throat to an OS-style shape. Maybe an hour of work for each horn. Besides the sharp angle, the squarish hole of the ones I got bugs me.
Aside 2: Tom Danley is maybe the smartest guy I've ever run into when it comes to prosound systems. No, he doesn't have a PhD, but he's equal parts acoustical genius, street-smart engineer and skilled garage mechanic. It's an awesome combo!