Interesting Tom, thanks. I thought it might be MDF.
It made me wonder what a "baffle" mod might look like. What comes to mind (for someone like he who has never designed a speaker and hence is naive bout it), is simply re-enforcing the existing MDF baffle from within the speaker, by attaching a solid aluminum plate to the inner face of the baffle (cut out in the shape of the baffle). Just to re-enforce stiffness.
I have no idea how implausible this is, or if adding any thickness to the interior of the baffle would alter other parameters (cavity volume?) that could throw things off sonically.