Here's one clue.
Atkinson's cumulative spectral-decay plot:
View attachment 121097
Fleetwood's compression driver with the type of horn that Jonathan Weiss despises (different scaling, but still...):
View attachment 121096
View attachment 121098
The resonances in the top octave are probably caused by the phase plug and the protruding roundover at the horn mouth > a source of diffraction.
"According to the Fleetwood website. The phase plug is said to prevent early reflections and aid dispersion".
Given the size of the phase plug, the latter only applies to the top octave and primarily resonances are sprayed at a larger angle (than below 10 kHz). On the positive side, this may contribute to perceived spaciousness.
Moreover, due to the small coverage angle (estimate: 60°), "the DeVille is relatively directional in the region covered by the horn-loaded driver".
In other words, a certain horn signature can be perceived on-axis. My brother also noticed this (without me saying anything).
The directivity is quite erratic - not only due to the suckout around the crossover frequency. This likely affects imaging, especially off-axis.
Whether these things are disturbing depends, among other things, on the room (acoustics) and the listening distance.
Although the woofer in the Fleetwood is far from bad:
View attachment 121109
I'd opt for a slightly bigger one:
View attachment 121113
Together with a different compression driver (all from the same manufacturer) and a better horn, the crossover frequency can be brought down to below 2 kHz, improving the overall balance. As a bonus, sensitivity increases.