Special Secret diaphram tweak
I am certain that I read here about a step in the production of ring radiators that one of the experienced assemblers developed a simple adjustment, (with a pencil?) on the diaphragms that allowed them to pass the factory frequency response and/or distortion tests. Apparently without this adjustment many of the drivers did not pass. If this is true, does it need to be done when a replacement diaphragm is installed? I figured it was something that might improve the OP's results.