My friend, Keith Howard, wrote me a wonderfully insightful note that I will share with you.
“I find it helpful to think of reflections as mono or stereo, in the manner of Manfred Schroeder when he performed his analysis of concert hall acoustics and confirmed the importance of lateral reflections.
Sidewall reflections are ‘stereo’ because they arrive at the ears from wider angles than the loudspeakers, so they increase interaural disparity (hence spaciousness). All other room first-order reflections (floor, ceiling, back wall, front wall) are ‘mono’ because they arrive at the ears at narrower angles than the loudspeakers and so reduce interaural disparity.
I’m not a fan of quelling side-wall reflections as you are, but if you do it then it’s essential not to mess with the spectrum of the reflection. Simple absorbers are bad news because they are more effective at treble frequencies than lower frequencies, so the spectral disparity between direct and reflected sound is increased, as if the reflected sound came from further off-axis. Why do speakers usually sound better pointed straight down the room? Because this results in smaller disparity than if the speakers are toed-in.
PS. Worth reading: https://www.aes.org/e-lib/browse.cfm?elib=3200 – published in 1979 but nobody took any notice!