How thick should the front baffle of speakers be?

brayeagle, that was a Warfdale in the old days. Cost too much to ship the things. 

fsonicsmith, it does not have to be eliminated. There are two types of distortion, Linear and non linear (this is psycho acoustic babble now, not my own) Linear distortion is alterations in frequency response which are very noticeable. Enclosure resonance would be in this category. Non linear distortion is IM and Harmonic distortion as well as mechanical distortion, buzzing, rattling and so forth. Studies (not mine) Have shown that Humans will tolerate non linear distortion levels up to 20% under certain circumstances the reason being that the distortion is being masked by the music. Non linear distortion is also more noticeable at lower volumes. This could be why some of us like higher volumes until system overload. This is the reason MP3 files sort of work. 
Frequency response is now completely manageable in the digital domain. You can correct virtually everything including enclosure resonance with room/speaker control. And, according to the psycho acoustic folks non linear distortion does not matter that much until it reaches ridiculous levels. Probably why we tolerate listening to such crude devises as loudspeakers. I guess their message is get full spectrum room control and be happy.......right.     

simonmoon, thank you for being the voice of reality here. Cheers.
The reason they used plywood back in the old days was that they had not developed MDF yet which is a far better inexpensive material to make loudspeakers from because of it's damping characteristics and higher mass. It is much stiffer than plywood. The old speakers were indeed musical instruments and colored beyond belief. Just listen to any Bozak B302A a speaker that was lucky in that it's colorations were euphoric as all get out. But some of the old Altecs were honky old boom boxes. Klipsch did a good job of controlling it. Higher mass and thicker stiffer walls are always an advantage however the speaker manufacturers have to balance that against shipping costs as for most it is a competitive market. People who buy Magico M7s could give a hoot.You have to pay a rigger to get the things into the house not to mention pray that your floor will support them.