Joe,
My room dimensions are:
Length: 3.65m
Width: 3.42m
Height: 2.41m
Quite small from an 'acoustical purity' POV.
The panels are done on the very cheap. Pine frames, medium-density rockwool (32kg/m3) on the inside. Not ideal insulation, but here in Australia we're unfortunately stuck with very subpar acoustic insulation products. I've been researching more however and our next set of panels (the 'prototype' models) will involve better (and safer) insulation.
The whole corner where the walls meet should ideally be treated. Ideally, you'd want superchunk bass traps, which are essentially a stack of insulation from floor to ceiling. If you can't do that, then simply straddling the corner with a thick acoustic panel gets close enough.
Behind the cab shouldn't be necessary. Just behind the mic and to the sides of it should cut it. If you want to go one more you can treat the ceiling above the mic, but as I was saying, try not to box the mic in, at the risk of creating a bass trap.
NICE dude, but it seems a bit dark, and christ you have those Mackies far apart! I like the artwork on that far-right absorber, and what are those corner bass-traps made of?