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

A couple of corner bass traps and some diffusion in the area between the speakers are what I would recommend. You want to avoid parallel walls front and rear so diffusion behind the speakers will help break up standing waves. 

@mashif thanks - note there is no rear wall per se, the area opens up to an open floor plan so the actual rear wall is 50ft from the speakers. 

That's good! If the low end sounds okay you might be set. Otherwise bass traps in the corners behind the speakers should help. 

I would try some acoustic panels, 2' x 4' mid wall behind the speakers.  It's a trial and error game.  My understanding is you want to prevent reflection off the front wall.  

“How important is it to treat the wall behind/between the speakers”

@macg19 

Back wall, along with front wall and side reflections are equally important for an acoustically well treated room. 

Ultimately, the character of your room—how live or dead it is—will be the deciding factor in whether absorption or diffusion works best.

Absorption panels behind and between the speakers can tighten up imaging and help tame those early reflections that smear detail. Diffusion panels, on the other hand preserve a sense of openness and depth, preventing the room from becoming over-damped.

A mix of the two strikes the right balance, but the room itself will be your ultimate guide, thus requires some experimentation on your part. 

In my own room I’ve found a combination works best (see my virtual system) …diffusion on the front wall to keep depth and openness, and absorption on the back wall and side walls to tighten imaging and control reflections. That balance really helped me achieve a more natural and engaging presentation.