First of all, just to mention this is only based on what I have tried so far in my pigeon hole .
Woods have the best in terms of dynamic , live , and airy (least pressure to ear). However , it wont increase much of your room size feel . Sounds will be more attacking and more punch . A bit downside is if room is small like me, you have more sense on visibilty of speaker presence .
Take note designs of diffusors ,placement (speakers and diffusors) all play some parts on the imaging too.
Styrofoams dont have the dynamics , live and airy compare to woods. In fact , sometimes I feel it is not live and airy at all because it seems like it stable the pressure feel. The only good side is it is much cheaper and can create bigger space feel than woods . It can also tame a bit of the highs(if speakers is too bright). Designs play a part but signature is there .
Foams dont have the dynamics , live and airy compare to styrofoams. It has the most pressure to ears too and may cause irritating at some occasion. However , it creates the most depth . Due to the pressure , sometimes using 2 small pieces can adjust the sense of staging in the room . Density, thickness and designs plays a part but sound signature should be same.
From what I feel , different materials have its good and bad . I myself like to mix and match + a bit of bare wall especially mine is small room . I tried to keep half of the materials mobile so once a while I will swap its accordingly to my taste on different recordings and etc . Or testing something.
(Bare walls have the most live and airy feel but messing up the details. But it is good to leave some places bare)
The most obvious to me is the speaker height to floor treatment as that is tweeter/ear level . A mix and match of materials usually can adjust the height of the imaging more effectively too . (Note: Different ear level you sit also can change height of imaging effectively)
There are small tricks to try out to increase the spacious of soundstage without changing anything . Try reduce upper mid bass to minimum and maintain deep bass. It should be able to increase the soundstage automatically . Ofcoz this is just for fun coz mid punch will be almost gone . If your testing same as mine , means it shows why different materials can create staging differently .
Another common way which most of you may know is by bringing your front speakers towards listener ( a bit like nearfield) .
Sit with ear level on the couch/sofa can increase soundstage as well. Ear above headrest will have more live and airy feel but soundstage reduce .
I would say anything can affect anything so just treasure its and play around for fun once a while.
Above just my fun testing during the years messing with treatments .
(***Different rooms may have different results so dont take my test results seriously )
Oh ya, I believe some may take out speakers’s grilles to listen before. The result is…(go try yourself coz FOC )