I think this is more of a problem pushing low frequencies, that compression ratios come more into play. On Tom Danley's BT-7 I think they have like a 3:1 ratio and when they were in development they had problems tearing cones apart at first, and I've heard other people talk about it when they start going over 3:1 ratios. On Ev's MTL's they crossover at 160Hz but I'm not sure what the compression ratio is on these, but they weren't ripping cones, just causing distortion to rise. After they discovered this they came out with mod kits for them to address the issue which I think was a 1/4" spacer between the 10's and horn's throat to reduce the distortion at the cost of 2-3db reduction in SPL's.

I've been following the threads on the new lab 12, but at first I thought you were talking about non-linearity in horns rather than the speakers vent hole.

Bill W.