John, what you have said here about Schoeps figure-8s is simply mistaken as a matter of fact, so perhaps you will want to revise the way in which you understand the issues which you are discussing. If the backplate of a single-diaphragm figure-8 capsule is acoustically transparent (e.g. Schoeps MK 8), that microphone's bidirectional symmetry can be as close to perfect as manufacturing tolerances allow.
On the other hand, if a figure-8 capsule has two backplates and they differ, its pattern will be lop-sided. How do you explain the fact that Sennheiser uses the two-backplate approach for their MKH-series microphones with all the other patterns, too, if this technique supposedly produces an ideal bidirectional pattern every time? Dr. Hibbing and Mr. Griese from Sennheiser gave an interesting paper at the European AES Convention in 1981 (available as preprint 1752 from the AES) about their reasons for using this approach--mainly, that it can reduce certain types of non-linear distortion in the capsule itself, as you mentioned.
I recommend reading this paper--you'll see that it isn't about pattern symmetry, which in practice is quite similar for all three of the manufacturers that you mentioned.
--best regards
P.S.: Incidentally, the dual-backplate, push-pull construction principle was introduced first by Schoeps several decades ago. They then stopped using it, and to the best of my knowledge, Sennheiser is now the only manufacturer of such microphones.
Topic: figure 8 mic for M/S stereo setup (Read 18086 times)