Diffuser or absorber behind listening position....and is it worth doing??

Erik, you nailed it; said what I was going to say and others had missed.

OP, for the same reason you know not to put the speakers too near boundary walls, you really don't want to sit right against the back wall, or even 2 or 3 feet forward.  You will hear mis-timed reflections of everything.  To a significant extent you will be listening to the back-wall as well as the direct signal from the speakers.

I agree with Erik, if you really must sit there then pad the back wall out all you can so it absorbs rather than reflects.

That is the only solution, but a crude one.  Unfortunately, 12x18 feet is not ideal for hi-fi, especially a big system. Whichever way you set it up, you are compromised beyond full salvation.

I have a similar shape room to yours, fortunately a few feet wider and I am set up the other way.  So along the long axis the (dipole) speakers can be 5 feet from the wall behind and I sit somewhere in the middle of the room, well away from reflections.  I have to compromise a little as the outside of the speakers are a little less than 3 feet from the side walls.  But for dipoles it's better to have a bigger space behind than to the side, as they face nearly straight down the room so most of the rear signal beams backwards.

MC has no clue on acoustics, to treat a room properly will require both absorption and diffusers. Glass is the worst for acoustics, no pods/springs will fix that. Go to the experts like GIK, ASC, or Acoustic Fields (as examples) to get their opinions. Once you treat your room, then you can apply tweaks to your system to get the best out of it, but you won’t be putting $1 springs on components to accomplish this. If you have a terrible sounding room, a $100,000 audio system will sound like a $500 system. 

Regarding the wall behind listening position. I support the use of absorption via self made panel.  One place to look for DIY material is ATS Acoustics. They are located in Piper City Illinois. Have a nice web site. They can place any photo you wish on to acoustic material for the panel. 

Good luck with this project.

OK, first off thank you all for contributing your insight and suggestions. Especially those that were considerate enough to circle back and answer additional questions!

So, I'm definitely ordering some absorption panels for behind the listening position to cover a 4x6 ft space and 4 inches thick. Main objective achieved!

In addition, I'll be researching a light weight diffuser that's practical to implement covering the T.V. located between the speaker on the front wall when listening (any recommendations?)

Lastly, I'm also strongly considering setting up temporary free standing acoustic panels ( I have photo lighting stands I could repurpose) for managing the side wall's first reflection pionts when listening as an experiment. I just need to determine if all the set up is going to be more of a determent or a benefit to simply enjoying music.....which is the really the point and don't want to lose sight of that.

Thanks Again

I have a room almost exactly the same size (11 feet by 17.5 feet).  Diffusers can be beneficial.  However, be aware that QRD type diffusers (quadratic) need very careful implementation.  QRD diffusers can do weird things to the sound if they are placed very close to your listening position (like within 6 feet).  In this situation, the QRD diffusers can boost some midrange frequencies while cancelling out other midrange frequencies.  This creates a very bad and unbalanced sound. (this was from testing QRD diffusers on the side walls).

I was successful in placing two 2' x 2' QRD diffusers in the middle of the back wall.  That was the only place these QRD diffusers really worked.

I also have two 703FRK panels in the rear corners and another two 703FRK panels mounted at an angle on the front wall / ceiling edge.  These panels are actually "reflectors" for mids/highs due to the foil on the front surface. 

The rest of the panels are absorbers or tuned membrane bass traps.

The variety of "diffusers" that GIK applies onto their panels are not really true diffusers.  They are primarily "reflectors" to prevent much of the sound from being absorbed, but still keeping the panel useful for bass frequencie absorption.

The GIK "gridfuser" is sort of a QRD diffuser but without the channel slots.  Their PolyFuser is like a broadband reflector that reflects all angles (sort of works like a diffuser but more forgiving).

Be careful with how much absorption you add.  You can definitely over damp a room and suck the life out of the high frequencies (thereby removing excitement).