Go dnewma04, go, educate his @ss!
Good choices on the plywoods you suggested.
I think the guys problem about plywood is that he is refering to the cheap crap at home depot that's like a penny per board foot.
Some people just don't realize that plywood was originally designed to be stronger than a slab of wood of the same dimensions.
And that it is stronger than MDF (being that MDF is basiclly just saw dust and glue).
My sub box is dove tail joints with biscuts and glue (only screws are helping to hold the plexi-glass in place), and is far more stable than glue/screws/& butt joints.