To Buy or to DIY, here is my question

really ? better than a cabinet within a constained layer cabinet with the outer cabinet wrapped in aerospace satellite grade CRFP, cured in an autoclave…. sure….

There is nothing more rewarding than designing and building something with your own hands.

ditusa,

Thanks for that link. Now there is one on the list. 

To build a speaker cabinet without it doesn't make sense. 

1. Stronger joint for structure integrity.

2. More bonding surface than traditional 45 miter.

3. Provides an air tight seal.

When I purchased my first locking miter bit I purchased it at Woodcraft. I figured I would ask the staff there as the majority that work there are seasoned woodworkers who have a shop. 

None of them had experience with it nor could advise. 

Personally, I would not build a cabinet without this joint. 

What's the point of your post?  In the aerospace industry, our typical markup was 5x-10x the parts cost to cover the rest of our operational costs plus profit.  Never begrudge a manufacturer just because you can't figure out what their cost of doing business is.

If you build the speakers yourself, you had better like them because you will find them difficult to sell. Unless you have a trade account, you will be paying a multiple of what the manufacturer pays for the components. There is also the opportunity cost of your time. The chances of getting better sound than a speaker from an experienced designer (sadly now deceased in the case of Pro Ac) who has spent decades refining his designs are slim.