I think we start by defining a monitor:
1). Two way design
2). Boxed for bass extension
Now we move onto the problem. This pic illustrates what we are trying to cover with just two drivers.

I think OB is out because it seems to always take at least 3 drivers to be able to play low enough into the bass region.
So the two approaches seem to be:
1). Use high quality tweeter like a Raal or Neo with a nice mid bass and design a crossover that can blend the two together as well as possible
2). Use a wave guide with a dome tweeter to play as low as 1000hz and then cross over to a bass/mid driver to fill in the lower octaves.
The first approach has air and speed in the top region that approach two does not have and the second approach has a coherence that the first approach does not have.
Building a box comes down to mass and internal design. I can't imagine anyone doing much better than Vapor in this regard. They have a speaker for design 1 & 2. SP Tech is built like design 2. The other approach is using the box to resonate. I can't think of any speakers that use this approach off the top of my head.
In diy, Dannys kit with the stacked laminate seems like the best way to go. Danny might have some new kits coming that raise the bar.
My only question is what about a airy tweeter with a waveguide can you have it all? None of these designs gets the ultimate in dynamics and sensitivity.
Seems like a bunch of tradeoffs.
My view from the outside looking in,
Robert