Now look at the real world. Even a very good horn has excursion peaks and dips as you pass through the bass range. The best you can expect is to reduce excursion just above the flare frequency and then again at about twice that frequency, maybe even a third excursion dip above that. But between these dips, excursion rises and it is at those points that you'll want to focus your design efforts. Those excursion peaks will be what sets your limits on the low end.