How thick should the front baffle of speakers be?

I understood that Harbeth's design goal was to make speakers that do not store the energy radiating from the rear waves of the drivers ( not the same as vibrations from the movement of those drivers). To this end the design is a success. The design would not seem to be as successful with the large woofered 40's.  I would doubt they intended to make their cabinets sing. 

Thinking Nicki Minaj, Taylor Swift ??

A singing cabinet is the natural and unavoidable byproduct of a thin walled cabinet. If this singing is pleasant or not is another issue altogether. Mr Shaw can can say whatever he wants about design goals and perceived sonic realities but the fact remains that this method of dealing with energy makes no sense to me and if you listen to a Harbeth speaker you may hear exactly what I am discussing. 

I have listened to the Harbeths. They are not perfect and I think there are probably some frequencies that do tend to excite the cabinet. On the other side these heavy MDF cabinets store low bass energy and then release it slowly with a noticeable dead sound, the kind I do not care for.