A single 18" bertha style cab is called an Emerald and when you put two together I think it adds a bit more punch than a Bertha given the added stiffness and material.
There are four of these in a club in Old Montreal, (unfortunately closed for the time being due to mismanagement..)
I def want to build four Emeralds as their sound is tremendous. loaded with 2242H.. A single Crown Macrotech 3600VZ is tight on the sub bass.
As for this config for Stereo Montreal
This was a reference class system. I can tell you that I had listened to this system loud for hours on end..
When I played on her at low volumes, the dynamics would fill the room, vocals were like the singer was in front of you.
The room had acoustical challenges but it was a giant beautiful monster & I miss her. this was an analogue beast.. Stereo is now a digital system.
If you want dynamic range this was the best system I ever heard.. it was slightly underpowered so at high volumes it would be slightly limited.
6 stack system btw.