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

Check out this room calculator to find your first reflections. Those in the horizontal plane and from the ceiling are more consequential than the floor. Good luck!

@macg19 in my profile's virtual system photo you can see my approach - two bass absorbers (8" thick) behind the speakers and a (home made) skyline-type diffuser between the speakers. 
 

I also have three 2by4 foot diffusers on the back wall behind my listening position. 
 

Vigorous thumbs up to the positive impact of sound treatments behind and between my speakers in a 20 x 26 x 12 ft. dedicated room. The sound stage deepened greatly to the point of an illusion of being able to step into it. I’ve sound treated the ceiling, too that further enhances the illusion. I use bass traps, absorption and diffusion panels. My speakers are floor standing Tannoys paired with Tannoy supertweeters and Hsu subwoofers in an analogue system, 

@bimmerlover 

Congratulations on putting together a wonderful system and equally complimentary room! 

@macg19 Congrats on a nice looking rig and room.  If you're open minded to a few suggestions for better sound, here goes . . .  these are not criticisms just ideas to try out.
> Early Reflection Symmetry: (1) I'd pull your speakers forward so that your soundstage depth may improve but also the carpet now catches the speaker's 1st order floor reflections. (2) Is the ceiling treated? Might you consider white colored Skyline diffusers made from Styrofoam at the 1st order reflection points so that they blend into the ceiling color wise and so that you aren't impaled should they fall? (3) Sidewalls are different construction materials and may cause havoc with tonal balance and imaging - try a stand alone 6" thick GIK Monster bass trap for each speaker's closest sidewall.  The right stand alone trap should be the same distance from the right speaker as the left stand alone panel is from the left speaker.  The farthest speaker's sidewall 1st order reflection (R_speaker -> L_sidewall and vice versa) may require asymmetrical treatment as pulling the speakers forward may now place the right speaker firing into the left doorway (essentially an absorber) so you may need to experiment with just a right sidewall treatment of another stand alone absorbing panel by the bay window for the left speaker's 1st order reflection or a reflective 3/4" thick reflective board to redirect away from you to acoustically mimic what the left doorway is to the right speaker.  (4) Place 1D QRD panels centered on the front wall and front wall corner bass traps
> have you access to acoustic measurement software (REW / OmniMic)? If so, then Energy Time Curve data for early reflections will help with trying to achieve acoustical L/R symmetry for 0-10ms.  I'd suggest frequency limiting the ETC data to 3 one-octave ranges centered at 500 / 1 / 2kHz as their reflection energy differs based on wavelength, speaker directivity, toe-in angle etc.  Focus on the 500Hz and 2kHz octaves for L/R cross-channel symmetry for tonal balance and imaging respectively.  You should also try to achieve cross-octave symmetry across the 3 octaves (500<->2kHz) for each channel separately for tonal balance purposes.