Lowthers and whizzers
There is always going to be interference between the wave coming off the back of the whizzer and the wave coming off the shaded portion of the main driver. This will result in muddied and uneven response in the critical human voice range.
I have never heard a whizzer cone driver I liked and then have never matched up to drivers with ultralight paper cones with controlled flexure. To my opinion they are a sticking plaster to make over heavy diaphrams work as fullrangers. An unacceptable compromise to my mind.
The only whizzer drivers I have used are Lowther DX4 drivers and I have tried them a number of ways. I don't agree with the hypothesis that the whizzer causes a muddiness in the midrange. If these drivers are used as widerange drivers, then reproducing bass is a primary reason why Lowther drivers can sound muddy. Removing the bass from Lowthers (and reproducing the bass using a separate woofer) helps significantly to clean up the Lowther midrange.
Using the Lowthers in backloaded horn cabinets makes the midrange muddy as well, most likely because the sound from the back of the driver is reflected quickly back through the thin driver membrane, and this sound is 180 degrees out of phase with the sound coming of the front of the driver. The compression chamber for the back horns can cause the Lowther cone to deflect which could also cause the Lowther drivers to sound muddy. Lowthers are much more clearer sounding when they are mounted on open baffle or in front horns.
Finally, I borrowed a pair of Lowther drivers which were treated with EnABLe. This made a substantial improvement in the quality of the midrange (particularly, less muddy midrange). I don't know if the improvement is mostly due to the dots added to the cone or the conformational coating, but Lowther drivers are much improved with EnABLe. But, these drivers still had the whizzer cones, and this seems to demonstrate that if used in its best configuration, the Lowthers are outstanding midrange drivers (essentially as good as anything out there).
I recall that there was a shootout between compression drivers in front horns and Lowthers in front horns, and most preferred the Lowthers.
Another data point, Nelson Pass once commented that the best midrange drivers that he ever heard are Feasterex and Field Coil Lowthers.
I can understand if you have had limited experience with whizzer style drivers why you would conclude that it could be the whizzer. Based on my experience, I suggest otherwise.
I think that if you made reference to the poor quality of higher frequencies reproduced by whizzer cones, which are sourced largely from breakup modes, you would likely receive little to no complaint. Although, I was surprised when I heard the EnABLe treated Lowther drivers, because the high frequencies were much better as well.
Retsel