Question: Sound Treatment behind the Listener?

How you treat your room is somewhat based on what type of speakers you have, monopole, dipole, omni. For example, a “dead” front wall is not ideal for dipoles. 
To your OP, we typically suggest wide-band absorption at the middle of the back wall about 4’ wide (maybe wider) with diffusers flanking left and right. 

You will get some reflection from the back wall but it will be low intensity as those waves will have travelled some 20 feet before they reach you.

Being of a practical disposition, I suggest you hang some heavy blankets on the wall behind you and listen to see how the presentation has changed and whether you like it.  If that may be too much damping try it with thin cotton sheets.  You will be able to gauge what is required and what suits your tastes.

Hi ozzy: Absorbtion panels for me work best at that position.

@ozzy why not reach out to GIK Acoustics

I’m working with Mike Major to finalize everything I need for my room and I find Mike to be very knowledgeable. Provide your room dimensions and photos and see what they say. 

I did reach out to Mike at GIK. Sent him pics of my listening room. 

He suggested, and I bought, 3 acoustic panels to go on the wall up and behind my listening position. Which was on a couch with a wall right behind it.