MDF came much later. Chip board was used a lot with vinyl wrap on lower priced cabinets and better cabinets used particle board with wood veneers like say JBLs of that era. A lot of manufactures with particle board boxes did use plywood for the front baffle as well. No doubt cabinets are made much better today with materials and bracing. Pretty sure the older Klipsch were plywood though and nothing wrong with that.
Quick Google search shows MDF was developed in the 1960’s. Be curious to know when speaker builders started using it. Possibly MDF came later when they started doing more internal bracing. My guess it was B&W, only because that was the first company I recall advertising the importance of bracing in their cabinets.
