you got to understand that diffusion will only work in a bigger context.
one thing is that propper diffusion would need deep panels that go down to 200 hz. those costs a lot and you would need A LOT of them. forget the little space between your speakers, a big part of the frontwall would need to be covered and the BACKWALL needs to be very covered. makes your room smaller. and costs a LOT. then, there is a physically defined distance from the wall to seating position. you need a pro to calculate it.
another thing, room treatment is also room, speaker and preference/music dependent.
there is a time difference between direct sound and reflected sound and diffusors might alter that to your dislike (or improve it to your liking).
a million people have bought diffusors but only a handful got true diffusion.if you want good acoustic, your fundament is bass absorbtion. you focus from 60hz to 200hz. it is safe as you cant really overabsorb.
then, if you want more, you got to think deeply about a concept for the entirety. whatever you choose, it has to be broadband. means if you choose absorbtion panels, they should be 20 cm think to go down to 200hz, otherwise they just create imbalance.
as for frontwall, there is no clear science. modern approaches go towards curved and wavy designs. diffusors are a bit outdated and some acosticians try to just scatter the sound behind the seating position on ceiling, sidewall and backwall towards the corners.
many including me, do belive that currently the best panels, respectively the best rooms worldwide are done by smt from sweden.
http://www.diffusor.com/PDF/Broschyr 2012 LAYOUT_komp wing sida.pdf
rather than a panel between speakers, my recommondation would be to buy 3 of the acrilic wings and place them right behind your chair / sofa, maybe half a meter. 10 times the effect and save to be on the good side.
you can add more wings on the first refletion points at the sidewall and later also on the frontwall.
Tutzing – Bernried | My-Sound
this room for example is all around treated with smt wings and then covered with fabric.