Room correction help

@millercarbon

Next most effective, 8-12" wide rectangles in the corners where the walls meet

You mean covering the corners like corner traps, the exposed face being 12" wide?

FWIW I experimented on first reflection points and I didn't hear much difference. Maybe these spots you mention male a bigger difference. I'll try them out. I really wanted to hear a difference, but didn't. 

A couple of things to think about:
normally you might want to place your speakers in the right location before applying reflection point panels. There is a rule of thirds or maybe 5ths meaning your should start your speakers 8’ out from the rear wall. This should help in bass response and depth. 
The space between your speakers seems it might be too large. This will hurt your soundstage and imaging. 
Check out the Cardas website along with many others that give you formulas On where to place your speakers based on your room dimensions.

I'll vote with cal18, although using an older Behringer 8024 vs. the newer 2496.  I've been doing this for...damn, nearly 3 decades, starting with an AudioControl C101 long ago and far away....

Never been one to clutter up a room with panels, traps, etc.  SAF is one concern, but her primary concern is being able to use the equipment without a degree in audio engineering. ;)

Rugs, curtains, and some 'tasteful additions' can go a long way.  All homes I've lived in have always been problematic acoustically.  The only way one can achieve 'Perfection' is to have the space 'built for purpose' which most of us can't achieve....

As Clint noted: "A man has to know his limitations."

The Answer: 2 systems.
-Main living space set up for Simplicity, but properly eq'd and capable of pleasing the both of us.
-My 'play space', which is just as insane as I might care to have it. *G*  (But capable of sending a feed to the mains in the living room...have your cake, and listen to it, too....*smirk*).

"Thinking outside the box" is easy.  Stop thinking there is a box to be outside of.... ;)

Diffusers and general room treatment are great, but I think you need to address speaker placement first; it's cheap & doesn't affect WAF. At 2.5ft from front & sidewalls may not be optimum. I recommend you try 2.5-2.8' from side walls & ~4.5 from the front wall and adjust thereafter for the best bass. Then place the flokati in front in-between the spkrs & sitting position -- or wherever the wife prefers (if it kills the sound she will hear it & correct).

Perhaps if you cannot here much difference there is a reflection that over powers the places you have tried . A mirror placed on surfaces should show you if you are accurately finding the reflective points in your room . After i switched to very wide dispersion components i got reflection . (077,2441,harstfield lenses.)
when i used stuff a touch more beamy i did not . (Cat eye 076, 2344)  its an ideal time to see if placement benefits your cause the most , then room treat( try the mirror to help guide )  , then position speakers again , you will never be done . This is a forever hobby .