Front or Back wall diffusers

yes, it can still have standing wave, nodes, etc., issues in the bass region. I apologize for the misspoken.

If we are talking front wall I would say floor to ceiling bass traps in the corners are first step. If we are talking back wall I would say absorption behind MLP is first step. Then you can experiment with diffusion. 

I would agree in that trend of treatment. The most benefit I have found in my room is from the bass trap.  I have high ceiling (10 ft) and also the WAF so I did not install the bass trap all the way but the immediate sonic improvement from only the back of speakers is noticeable, less reverbration and cleaner vocals.  

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: