A lot of info on this subject can be found on the Planar Speaker Asylum site. Magnepan is by far the most-covered planar brand there.
Magnepan Mylar Repair
One of my MMGs has started doing the bass slap and I think the issue is the mylar has stretched. I took off the sock and visually nothing looks wrong. Does anybody know how to re-tighten the mylar? I contacted Magnepan and they are only doing warranty repairs for the foreseeable future.