Front or Back wall diffusers

The need for acoustic treatments is room dependent and I just share my experience for whatever it is worth. Three critical elements are (1) diffusers on front wall, (2) bass traps on both corners and (3) the distance between the speaker tweeter and front wall. (3 ft as a rule of thumb) As a gentleman mentioned earlier, you do not want the reflected signal be perceived as a direct signal and mess up the image of sound. If you need to place speakers closer to wall, some form of absorbers (like curtains I used) behind the speakers is also recommended. As far as the back wall is concerned, if your sitting position is cleared roughly 1/3 of the room from the back wall, the treatment behind is less critical.  Lastly, do not overdamp your room!!

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

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.