I should mention that my interests tend toward pro audio/live sound applications.

For want of a better phrase, the "smoothing guide" was a feature I noticed in an EV vertical line array. Two 8" cones were baffle-mounted at the back of a 90-degree conical wave guide, with a flat panel 15" driver on each 45-degree horn wall. A formed plastic or fiberglass device (not a phase plug) was mounted over the 8's with curved surfaces to smooth the transition from the rear panel to the horn walls. Most likely, it was intended to minimize reflections of the higher mids. So much for that!

So, if the ol' brain sponge is soaking up terminology correctly, then the baffle-mounted (what's the official title?) driver will yield a lower cutoff due to beaming than other types of throats, yes? And, the wider the horn coverage pattern, the lower the cutoff becomes? In this case, the coaxial HF section would be a large format horn/compression driver combo (strictly active XO).

Gotta finish this in another posting, time to pick up the Wife from work! Thanks, man!