Dave,
I don't doubt the benefits of backplate coupling, especially when it allows you to flexibly decouple the woofer baskets from the enclosure. Short of emperical testing/comparison, I won't be able to prove or disprove my inclination that good between-driver bracing can be as quiet as backplate coupling. So I'll give you the benefit of the doubt and trust your ears over my hunches.
*However* your caveat about woofer linearity is a giant variable. You reference the Lambda drivers, which are some of the very best in the linearity department. I'm suspicious that almost anything less would benefit from push-pull alignment, especially if called upon to perform significant excursions. Along the continuum of declining driver quality, there is some crossover point at which your coupled push-push idea gives way to push-pull, in terms of what yields the greatest benefit. Where that point lies is a hard question.
Wouldn't it be nice to have both?
I just had a eureka moment--a vision of the best of both worlds:
What if you performed a dustcapectomy on one woofer, and then solidly coupled the exposed pole piece to the backplate of the second woofer by way of a large hardwood dowel, or some other extrememly stiff cylinder? Viola! A firmly coupled push-puller!
You could bolt the cylinder to the pole piece with a lag bolt through the vent. Actually, you could decapitate both woofers and bolt from both ends. Then you could stick a phase plug on the outward-facing woofer. Who needs duscaps anyway?
Bill
