How thick should the front baffle of speakers be?

3/4" plywood didnt work just fine. Resonance is a bad thing. Think of the surface area of a large speaker cabinet much if not all of which is vibrating. 

I would think that aluminum would sound better due to the simple fact that it is roughly 3 X stronger than wood. I would also think that equal parts of aluminum would be much heavier. 3 X the wood to equal the strength of aluminum would still be lighter than the aluminum. I dont think aluminum is used because it is cheaper I think it is used because it is much stronger and less material is required.

Well I dont think a speaker can sound accurate with one genre of music and not another. Harbeth's lossy cabinet represents the worst possible approach to this problem in my opinion. 

Harbeth owners no doubt like the way their speakers sound, but on the larger models the resonant cabinets impart a sound that isnt on the original recording. All speakers do this, but on the Harbeth models I have heard this flavor is very apparent. So I naturally object to those that think these speakers sound natural as I hear the extra "gravy" as a distinct coloration. Just my take.

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.