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for the enclosures there have been two approaches
1) 3/4"mdf bonded with 1/2" plywood; or
2) 2 layers of 3/4" mdf bonded together
- can you tell me the pros and cons of each method suggested?
The goal is to make the cabinet very rigid, to get good clean bass. Thus, either approach should be fine for that goal. I chose the 1/2" plywood (basically, I did what Wayne did), and it sounds great. I can tell you that the completed cabinets are each nearly 100 lbs! If you use double-thickness MDF, they will be significantly heavier than that.
Off the top of my head, and without thinking it through carefully, it might be easier to construct the double-thickness MDF cabinets, since you would not have to carefully measure and place the plywood (which was a minor hassle, especially with the braces).