ydjames - the breakup you describe can be caused by various things. Among them could be lose voice coil turns or other non-design-related phenomena. Let’s rejoice that the new drivers fixed the problem.
Last week I had a ’sound’ in a CS1.6 woofer similar to what you describe . I went looking for ’dirt’ in the voice coil gap, but instead found a steel washer resting on the ’top plate’. That little washer had fallen out of the driver mounting boss and stuck to the driver. It saturated and caused distortion in the magnetic flux field. In my case the ’fix’ was easy. Remove the washer and the distortion is gone.
Follow-on is that I am replacing the steel threaded inserts in the driver basket and the steel mounting bolts with brass, and the washers with fiber. I expect to hear the cleanup of a far lower level of that same distortion. That hardware is only an inch or so distant from where that single washer caused audible distortion. Speakers can have lots of low-level anomalies and sub-optimal implementations. Some get identified and fixed or improved during development, some are worked out during future product iterations. Some still remain to be addressed.