I made a few more measurements today with the diffusers in different locations, I had one unexpected response, it surprised me enough I ran it again, here they are:
Left Speaker without Diffuser
Left Speaker with Diffuser at Window
Left Speaker in Front of Rack
Left Speaker Overlay
The reduction of the 40Hz peak is what surprised me here, I didn't think the diffuser would have any impact on these bass frequencies. I measured it again to verify, same result.
Right Speaker no Diffuser
Right Speaker with Diffuser at Window
Right Speaker with Diffuser in front of Rack
Right Speaker Overlay
I also tried moving the mic from my current listening seat to several locations in front and behind, it became obvious you would have to tweak the setup for each location as the suckouts and peaks changed with each position change, regardless, I am limited within a couple of feet where my listening seat can be located from a practical aspect.
I also moved the mic to about 3' from each speaker, here I noted a difference at high frequencies with the mic pointed at the speaker vs having it point up. For the rest of the measurements the speaker was pointing straight up.