Coated woofer - why?

Coating a speaker will reduce the amplitude of cone break-up and the associated resonant behavior. Especially in the upper frequencies of the drivers range.

Those KEF drivers were coated with a doping compound called plastiflex, which did exactly that.

Smooth response and low coloration are the audible results.

Let me add, since coating the woofers was done by KEF as part of their construction, I am not sure you could call this a "mod".

Back in the days when i made my own speakers I used to do it to add mass to lower the FS (free air resonance) of the driver and to stiffen the cone.
The way it was done to have a nice ripple effect is to face the driver upwards paint it with thick coat of dust cap glue/Aquadhere (a clear drying wood glue) give it a 25hz sine wave single so it’s wobbling nicely, this gives the glue the ripple look while it sets, 20mins or so. 

Cheers George