The aquaplas coating is intended to damp resonances within the titanium diaphragm. I'm guessing that the ribs, while stengthening the diaphragm at high output, create phase anomalies of their own due to the ridges in the otherwise spherical surface. It was probably not felt to be an issue in pro applications, but became a concern in hi-fi use. FYI, JBL was using the aquaplas coating on their ribbed pro titanium diaphragms for a time, but I don't think they are now. I'm not sure why they stopped.