Going lower than the rated crossover frequency of a horn

From my experience, there is no issue using a compression driver down to horn cutoff in a domestic setting.
Radiation pattern will widen well before horn cutoff but match it correctly to a woofer of the right size and you're all good.

For instance, I use a Ciare PR614 which provides gain down to about 550 Hz even tho the radiation pattern starts to widen below 800 Hz.
Crossing to a small woofer at 550 Hz, the radiation pattern is more or less matched.

Distortion obviously starts to rise quite fast below actual horn cut-off but a steep filter fixes that.
See attached measurements of a small horn (STH100) vs a big horn (H812) and corresponding distortion plots.
 

Attachments

  • Screenshot 2023-02-01 at 23.31.03.png
    Screenshot 2023-02-01 at 23.31.03.png
    385.8 KB · Views: 86
  • Screenshot 2023-02-01 at 23.31.16.png
    Screenshot 2023-02-01 at 23.31.16.png
    306.4 KB · Views: 88
  • Screenshot 2023-02-01 at 23.31.22.png
    Screenshot 2023-02-01 at 23.31.22.png
    342.3 KB · Views: 83
"Crossing [..] at 550 Hz"

Yes indeed. Home use (low SPL) + a steep filter makes it work just fine.

Some TAD speakers (e.g. 2402) used a CD crossed at 650Hz, 2nd order, and they didn't use a particularly huge horn (the cabinet was only 60cm wide).

---

My own preference is based on trials with less stratospheric $$ 2" drivers like these.

https://usspeaker.com/B&C-DE950-1.htm

http://warehousesound.com/jbl2445.php

...and they put me off the idea of using these big format CDs over a 5 octave band. Their top octave is all "secondary resonances" (JBL Technical Notes Volume 1, Number 8) and I don't like it.

You probably chose more wisely than I, by starting with 1.4" exit devices.