Front or Back wall diffusers

Which walls are the front and back. 14ft or 24ft ?

Do you have windows on any of the walls ? 

Floor to ceiling bass traps as others suggest , then in my opinion buy 2 @ 2X4 2" thick absorbers  and 2 of the 2D or curved diffusers , then place them in the front middle and side walls , then swap them . this is the only way to determine what sounds best to you . Then you can add more as needed or wanted .

One thing that hasn’t been brought up is measurements, if you measure it helps confirm your choices. You don’t have to get "technical" to measure. I got a PW Link preamp from paradigm with ARC room correctionfor$199. I use the digital out from the Link into my dac, and presto, check out the before and after. The before graph is already pretty tight because of my room treatments, ARC just dialed in for that last bit of OTT clarity:

The main difficulty with setting up diffuse panels are you do not want them too close to your listening position. Six feet away is the minimum and at least eight feet away is the preferred. The two biggest issues center on the size of bass waves and the distortion of early reflection points. Diffusion of sound using panels is a luxury most people do not have due to the distance requirements. I use two 3D diffuse panel behind my speakers, on the front wall. I sit too close to the rear wall for them to be of any help, so on the rear I use absorption panels so I slow down those reflected sound waves and give the room an illusion of being longer than it truly is. The diffusion panels on the front wall help distribute the sound bouncing off that wall so it becomes even more delayed before it reaches my ears. The idea is to delay and scatter the sound waves around the room.

Talk to GIK.  They’ll give you some good advice given your room and offer products that are relatively reasonably priced. 

IME GIK has great products that I purchase....but their advice has been the same as other pro-audio panel makers...and thus bad for hi-fi....which is to load the room with absorption and create a dead room with low reflections....making the room sound smaller and closed in.

With hi-fi, you don't generally want to absorb the first sidewall reflection unless your speaker is very close to the sidewall...this was first discovered by Floyd Toole during the Harmon listening tests that determined preferences. 

@seanheis1 @axo0oxa I’m seeing several people here advocating for diffusion over absorption.  Are there any general rules as to when/where it’s better to use diffusion or absorption?  I’m very interested and not clear on this at all.  My guess is it’s largely room, system, and personal-taste dependent, but just wondering if there are any generally accepted rules on this.  Also surprised someone argued against bass traps that I’d never heard before and had me scratching my head a bit.  Thanks for any thoughts and/or hard-won personal experience.