I think one needs to qualify the statement you quoted. Sure, regular AC grade softwood ply is not a great choice. But we are talking about 13 ply void free hardwood birch plywood in this thread; a different animal.Plywood will vibrate more than MDF and allow more midrange energy to bleed out of the cabinet through the walls, which is bad for the sound of the speakers. Hence the suggestion to use more bracing and dampening materials.
Not going to argue with anyone else out there, this is just somethign I came across..
I guess I'm just playing Devil's Advocate. :devil: