Room Treatment? How important is it to treat the wall behind/between the speakers?

@macg19  You're welcome. Just so you know I prefer OmniMic to REW but folks can send me REW data in a text file format to analyze.

Macg19, as you decide what treatments to place on your sidewalls (and front wall 1st order reflections), Energy Time Curve data for the first 10ms will help guide you towards symmetry both cross-channel (Left vs Right) and cross-octave (across several octaves but within the same channel) for tonal balance and imaging aspects.  

Here is a post I wrote somewhere else about "Before" vs "After" sidewall treatments that goes into a lot of depth and analysis - maybe too much for most - but shows the level of detail that can be used to guide the process. Beside the original post, I added two more comments (further down page 1 and on page 2) as there was too much to include in the original post.
https://www.canuckaudiomart.com/forum/viewtopic.php?f=4&t=69938

I'd put a panel on the two side walls in the upper corners, where you have two untreated parallel panes that are bigger than 2x2 feet. A lot of reflected ping pong is going on there from the way it looks (especially in the 1k -2k range based on room size). Since the speakers are near those corners, any omni energy from the boxes is going to hit those upper corners first.

Even a 1.5 x 1.5 panel on each side would help. If the room look is an issue make them the color of the walls so they blend in.

Anything to keep them from being flat would be better than nothing. It's not so much about 'reflection' in this case as room resonance between those two open areas.

Every speaker is different. Needs a different setup, needs different room treatments, to get the best out of it. Refer to manufacturers recommendations and go from there. Easy.

@squared80 I wish it was that easy. The user manual is generic and basically says use the trial-and-error method.

Speakers are far more predictable than rooms IMO.

@nsh123 I was able to easily test your suggestion by stacking a 2x4ft panel on top of the sound blocks. It made no measurable difference (frequency response only) and I didn't hear any difference. That doesn't mean it might not be a good idea once I get the rest of the diffusers installed and get to measuring properly.

 @macg19, There has been some good advice here and I think the take away is the fact that  room acoustics are paramount which is a thoroughly understood subject requiring measurement to remove any guesswork. Fortunately you are in possesion of REW and a microphone and you also have the generous offer of someone experienced to analyse and help you understand what you are looking at. 

Your room as it stands appears to be lacking in absorption. The slatted wood panel on the left wall will help somewhat but it does not represent broad-band absorption which is needed. This is where measurement helps. You will have a visual graph or plot providing information on the decay times across the spectrum indicating what frequencies are the most troublesome. This is reflected in the RT60 info which is the time it takes for the sound to decay by 60dB. Some prefer to work with RT30.

As mentioned this is a scientific endeavour and the suggested RT will differ according to whether the space is a school room, a church or large hall. These times can be found online but for the average sized domestic is about 300ms.

Your room will be too lively as seen in your photos. Consider adding more drapes/curtains to the right wall. Try hanging a duvet or two over the existing curtain rail to hear the effect of adding absorption. Also listen through closed back headphones with your main speakers off to get an idea of the damaging effects of strong early reflections and room modes. If there is no bass source there are no room modes.

Another and IMO excellent treatment is look at the ceiling for diffusion and absorption. The thin carpet you have will only absorb a narrow band of high frequencies. Having 4" or better 6" thick absorbers work very well not only because of their function but because we have evolved to allow for floor bounce.

There is something else that you can do and that is to directly address the room modes by adding at least 2 subs. From experience and the work of professional acousticians it is the best weapon against room modes. These modes can be seen in the plot you provided. The peaks lead to muddy and indistinct bass and the null between to loss of bass information, that is to say missing important bass which is the foundation of music. Sort this out and you will get to hear what your nice speakers are really capable of. If you go this route, which I very strongly encourage, get subs that are sealed which means no ports or passive radiators and most importantly to have continuous phase adjustment which allows for placing the subs almost anywhere you like. Subs that only provide 0-180 degrees phase flip complicates set up and usually require placement somewhere inconvenient.  

@kevinzoe, which version of OmnMic do you use? I too use OmniMic the original version but I’m interested in the new version which provides time of flight for speaker design. Curious to know your thoughts on the new one. Interesting room you have there, congratulations.