The wall behind my left and center M80s is about 8’ farther back than the wall behind my right M80. I would love to be able to put them right under (flush with) the screen but if the left M80 isn’t pulled out so that the drivers are at least 3’ from the wall immediately behind I notice a difference in the imaging.
Also, when I had my M22s set up as wide speakers the drivers were a little farther from and angled off from the walls and my Onkyo would set their crossovers at 60Hx. I’ve got them up against the back was as rear speakers right now and the auto-setup is setting them to 40Hz crossover.
In my apartment when I had my QS8s corner loaded my Denon was setting them to 60Hz. Now the QS8 I have flush with the wall are being set by my Onkyo to 80 Hz while the one on stands out further into the room are being set to 90Hz.
So it seems to me there is a measurable effect going on, however, except for the imaging issue cause by putting one of 3 M80s close to a wall and corner loading the QS8s I never noticed any effect with my ears.
Given the boundary reinforcement I was getting with corner loaded QS8s I imagine if you put a rear ported M80 in a recess even if it had a few inches around it for the sound to escape that the reverberation of the sound in the recess before I escaped would play havoc with some frequencies. Most recommendations are not to even put “bookshelf” speakers in an actual bookshelf especially if they are rear ported.
