How thick should the front baffle of speakers be?

I've heard the Concerto 4 speakers at an audio show!

They are extremely good, for sure. When I heard them, knowing what I do about speaker cabinets, I made notice of their cabinet construction, and how that contributed to their sound quality. 

The rear mounting with beveled edges is a waveguide, which is a type of horn. 

More traditional horns, tend to be more concerned with gain as first priority, where waveguides tend to be concerned with controlling the pattern of dispersion as first priority. 

Whatever happened to those things you would stick to the sides of your speakers to deaden them? Did those pass into tweak history? (I don't really care) Note that my current speakers, Klipsch Heresy IIIs, tilt back so anything placed on the top just slides off. Keeps things from gathering there.

The thickness of the front panel should also be determined by how wide the front is. A large 16" panel will necessitate a stronger panel material. Narrow front panels not so much. Plywood is good, better then mdf. Alon  Wolfe went to plywood on his earlier versions of the Magic monitor. 

" wheres the proof "  Simonmoon is 100% right and that has been my experience too and measurable through REW and TrueRTA but you can't post results here to prove this because no pictures allowed.
  I had a set of La Scalas that had been damaged and I decided to replace the 18mm sides with 25mm Baltic Birch sides. You don't have to believe me but the perception of much lower bass was startling. What I was actually hearing though was bass without cabinet resonance masking it and it was a real eye opener. Since then everything I build with any area at all gets 25mm wood. Real honest wood and not MDF.