Room treaments

What type of speakers, size of room? Diffraction might be better at first reflection or nothing at all. 

Each room is different so IMO it's best to experiment a little first (location & type) before properly securing the room treatments in place. Also, noromance is spot on about using absorption type panels... A little can go a long way. I use a combination of diffusion/absorption on the side walls & behind (between) the speakers. Absorption on the ceiling & some behind me. Floor is slate with a leather rug between the speakers.

The room is so important to overall sound quality.

Have fun with it, it's certainly a learning experience.  

My room is 12 wide by 18 long and ceiling is fairly low at just under 7 foot
I built the room specifically for my 2 channel system .
I have Salk Sound SS-6M 2 way monitors with all YBA solid state seperates
I will play around with absorption and difraction/ diffusion.
I made the diy 2 by 4 sound panels and will play around with placement but I was thinking more absorption at first relation points rather than diffusion.
On the back wall behind my listening chair I ideally want both absorption and difraction so I might experiment with some DIY diffusers and maybe a home brew quadratic diffuser. I have seen a few designs on you tube that I might try.
But as far as corner treatment I haven't given that much thought. All the bass traps I have seen take up a lot of re estate. 
I guess it is a fine line between a lively room and a sonically dead room.
I appreciate the advice 

I have a theater room your size.  I added 4x8 wall panels from 4seating.com.  I have plush carpet on the floor and nothing on the ceilings.  Believe it or not it is almost to much.  It is great for the theater room but not for a two channel room.
I just added cinema round panels from Vicoustic Acoustics in my very large great room where my two channel area is.  My room is 25'w by 40'L with 16' ceilings.  Just adding 4 2'x8' cinema round panels helped my room substantially.  Vicoustic provided a whole analsis of my room after providing them a video.  I have used GIK and Audiomute.  With your low ceilings, I don't think you will need that much.  Do it in stages....see how it sounds.  

Reach out to GIK Acoustics for one on one free consultation. Once they make their recommendations, do it in stages. I did my room in three phases!