Pretty much nailed it.
The boxy aesthetic and recessed baffle is a diffraction nightmare, just look at the jagged response. A flush baffle with rounded edges would greatly improve that.
Even modest internal bracing would quell cab resonances.
Another vestigial quirk is designing specifically for corner placement, which may have been customary in 1953, but will result in inadequate bass and tipped up response if you place them away from walls or up on stands. So throw in adequate bsc, and tame those unruly mids while you're at it, to get a more linear, room- and ear-friendly tonal balance.
How about the directivity match between the woofer and mid horn? Maybe a smaller diameter woofer, larger format horn, and appropriate crossover frequency can achieve that. Heck, why stop there, let's simplify things and make it a two way.
I think the results would be very much like the Jeff Bagby designed F12 Tempest kit speaker. They're just as sensitive as Heresies but otherwise vastly superior in every which way. Why Klipsch doesn't employ similar basic improvements is a mystery. I think a better performer might actually sell better.
@MrBoat built a schnazzy pair of Tempests, and isn't too far from
@ErinH . I would love to see a full suite of measurements from those just to compare with the Heresy. Battle of the Garage Speakers! (They are no longer available in kit form, so probably not of much interest outside those of us who built them.)