Thank you for that post. The problem is those speakers, quite clearly revealed in those measurements and the design choices the designers made.
The problem is one encountered far too often in three way speakers.
The root cause of this failure is an inadequate mid range driver.
This really is an object lesson in what can go wrong if you are not careful.
The problem is that null between about 150 and 300 Hz or so. That is actually an octave.
This is a really important octave and a major power band, especially for orchestral music.
The cause of this is the crossover points, at 275 Hz and 2150 Hz.
The problem is that that midrange driver does not have nearly the bandwidth adequate to be a decent midrange driver.
So a low passive crossover at 275 Hz was chosen. Passive crossovers below 400 Hz are really bad news. The components both inductors and caps are just too large. I learned in my designs to not design a passive crossover below 400 Hz a very long time ago. Even there, they are tricky.
This low crossover was necessitated by the mid range driver having inadequate top end extension. A decent midrange driver should be able to reach 3,500 Hz and preferably 4000 Hz. This allows it to cover the speech discrimination band and the crossover points not suffer from stacking.
If you have mids that require a cross below 400 Hz then that speaker needs to have active electronic crossovers and at least two amplifiers.
Unfortunately so many mid range drivers really are not, and do not cover anything like the midrange bands. In this speaker crossing at 275 Hz I consider the bass range. In my book bass is up to 350 Hz at least and I consider midrange the speech discrimination band from 400 Hz to 4000 Hz. or at least 3,500 Hz.
There really is no easy remedy for your problem or actually at all as the FR of the mid is essentially too narrow. However removing the low pass to mid crossover and replacing it with an active crossover and bi-amping it would help. However that is a major engineering undertaking which I feel the quality of that mid range driver does not justify.
Unfortunately your only viable solution is speaker shopping.