Two surprising things I found that improved my imaging and staging...

Regarding rear wall and stuff between the speakers. I find it highly dependent on the speakers. My ribbon speakers didn’t seem to care that much about low racks (2.5’)  in between and liked the bare painted sheet rock behind them. My Sonus Faber Amati Traditional sound much better without the rack in between and with a thick Afghan rug covering the entire wall behind the speakers. 

Also, while the rule of thumb with my speakers is to toe them in to converge ~18” behind the listener… they sound much better in my room with no toe-in what so ever.

So, for me, it depends.

"It depends" is the fundamental law in small room acoustic for me...Because we must change a room with our ears and for them when listening a SPECIFIC speaker in the room...The frequencies response of the room in the vocal timbre bandwidth is the pilot guide to do this...You "bent" the room to what is better for your ears listening human voice with an active mechanical modification of the pressure zones distribution grid in the room not only with panels reflective, absorbing and diffusive in a passive treatment......

Why human voice?

Because the inner relation we have with all acoustical cues linked to the timbre of a human voice and his MEANING for us...All of us we can identify a correct timbre FOR EACH ONE OF US but which will not be "exactly" the same for none others save us...Then our small room must be fine tuned for our own ears/speakers specifically for a better human voice and instrument recognition when playing a note...

A great musical hall is not a small room....Acoustic laws cannot be used in the same way in this 2 cases....For example the use of reverberation time and the timing of the different first wavefronts cannot be the same in a great Hall and in my 13 feet square room...

No recipe works well for all small room because of their different geometry, topology and acoustical varied content...

No recipe will never compete or replace fine tuning of the small room...

But at the end this fine tuning of passive acoustic treatment and active mechanical control of any small room is the greater of all audio possible improvement with a specific pair of speakers...Greater than most upgrades of gear ....

Small room acoustic must be designed for specific speakers in specific small room...

A "tweak" like putting a rug can help, but we are short of a true fine tuning with some easy recipe of this nature only... Fine tuning is related to listening experiments and time but at possible low cost in my experience....

And any electronical equalization is a TOOL, not the solution at all...

For sure a dedicated audio room is the only essential  luxury in audiophile world...

I am not a scientist, but it is my experience with my room...

I have found HIGH CEILINGS and LACK OF CARPETING just won't allow you to hear what your speakers can deliver. I've been in both low and high ceilings... carpeted and tile covered flooring. Huge difference.