Do you have in space measurements you could share by any chance? A sort of comparison between the datasheet and actual real-world performance?
Here's a set of indoor ground plane measurements. Never got outside with this one....
Blue and red are the two sections raw measurements, and green is processed.
Also, can you see some kind of horn structure under the dustcap? If so, does it seem to protrude out beyond the woofer cone or does it transition to the woofer cone?
(obviously not asking you to tear apart your speakers lol, just any visual observations would be appreciated as I try to identify the source of potentially jagged response - the sort which seems to be lacking in that B&C driver...)
Whatever horn structure there is is fully behind the dustcap, so sorry I don't really know much about it.
I didn't spend a lot of time on this build, because I dunno, it sounds fine, but sounds like PA. Not like the hi-fi sound I'd been getting from more typical CD+ horn + separate cone driver builds. It has a lot a small ripple like TimVG mentioned with his build, which simply can't be filtered out.