What a shock to discover what I did with this speaker. Usually resonances color the sound. They don't become instruments on their own. But that is what happened here. And in a design from the talented Andrew Jones. Given how easy it was to detect the issue in multiple measurements, it should have been caught and fixed.
The control measures seem to fail sometimes. I had a pair of Elac Navis (Elac Argo before the naming issue) speakers when they first came out. One of them was fine. The other one didn't have a sealed box behind the tweeter/midrange and you could feel air coming out of the gap. Maybe that would also change the woofer behavior having a larger, extra unsealed box to work with.