I was hoping Tractrix might defy some of that, but it looks in theory like the horn and driver throat size must be precisely optimized for a successful two-way design with extended HF.

The other thing I'm observing is that LE85/HL91 isn't all that flat in frequency response through the region they're commonly used here. I'll measure a bunch of 'em before concluding that, tho. Could be they'd benefit from a notch or two.

I don't have an agenda in this; I'm just following up on what's appearing in the routine course of doing these investigations....