I would never argue the merits of MDF, Press-Board, or whatever. But keep in mind the cab is 40-45 years old. They simply did not look THAT far ahead.
Anyway.....
...yeah, great choice to do it right with ply. It will last another 60 years, or until the rest of the cab falls apart.
I have a real liking of Eminence speakers. The Texas Heat is one of my favorites. Do not think I have ever read a guy say he did not like the C-Rex.
Are you going to run them together.?
I have always had a soft spot for mixing speakers as well...
best