Interesting program, but..., the way I read it is that at 100Hz your deepest well would be 1.5m (or more) deep.
The first thing you need to do (me thinks) is to find out what the ofending frequency is. Than, scout the room for panels/doors/walls/cabinets with sympathetic frequencies, and eliminate those with bracing.
And than, if there is still a problem, use Helmholtz. Sorry, no shortcuts. E
That size of diffuser won't do a single thing to the boom of your room. Nothing. It's not going to change any wave longer than a foot. Last I checked that's 1KHz and higher.
All that diffuser in your front soundstage is going to do is confuse your directionality of source.
If you want to control the boom of your room you need something to absorb the low frequencies. This involves bass traps (helmholtz are popular), very very thick (like multiple feet thick) absorbers, or some diffusers that have a depth of a few meters (though this won't actually absorb anything, so it's probably not what you want).
Physics baby, physics. A lot of "Audio Technicians" forget to check physics.
I just read through all your links above and it looks like you're well on your way to a nice sounding room, but it's understandable that you're getting a very boomy sound currently as you're using a lot of ~4" thick rockwool. This will absorb reasonably down to the mid-bass range but then does nothing for the low frequencies. Therefore you're sucking out all the energy except the bass, and you live in a brick building so the bass then just resonates around the space. You need something to suck bass up so that all the frequencies are evenly treated.
You may find some luck in moving your absorbers slightly out from the wall 4" as this effectively doubles the thickness of them. Still your low bass need a specially designed trap. Start reading up on helmholtz resonator traps. They can come in many different sizes and shapes, and you'll be able to turn one or two of your panels (possibly the picture frame one) into a helmholtz quite easily with a piece of pegboard or custom-drilled rigid plywood (better).
I should say your diffuser looks great and I certainly wouldn't scrap it as your room appears slightly over absorbent if anything now. Some live acoustic diffusion is certainly needed or you'll sound like you're in a dead space. However you may want to double check the exact frequency range that the diffuser will affect and then think to yourself: Do my directional tweeters even send any acoustic energy in that range behind them? It may currently be doing little more than looking pretty.
Cool blog too.
Just keep in mind as you're tweaking away that bass frequencies have ridiculously long wavelengths compared to your room. To tame the boom you'll need big well designed things. Also, don't take the spec sheet for absorption coefficients too seriously below ~400Hz as this is not the intended range for this product and the measurement techniques to get those numbers are often fatally flawed (if 1.0 is a perfect 100% absorber how do they quote numbers above that as how much they're actually absorbing...). Get creative with sections of your room that you don't use - can you stick a helmholtz resonator under your couch, in your coffee table, behind your speakers, etc...