When I experimented I found in my room of 14' wide that both diffusers and panels with scatter plates placed on the side walls had a noticeable effect which I did not like - I assumed they were too close to me. I now use absorbent panels.
These days I have my gear on the side wall so I place the panels close to the speakers. One panel covers both first and second reflections. I find that 242 panels best because they start absorbing higher up the frequencies (compare test results on the GIK site). I also have them on the wooden feet, which again I found sounded better.