One speaker observation from the New York Hifi show.

Regarding the wall behind the speakers. Regardless of the type of speakers the wall is a reflective surface. There will be a time delay between sound from the speakers and that from the wall. This is Distortion. If you want refined,delicate,nuanced sound,at all volumes, this distortion Must be avoided. ie keep speakers away from that wall. I like a minimum of 40 inches; and use diffusion as well to make the reflected sound-noise- as diffuse as possible. 
Re “Toe in”. Take a look at any data on speaker measurements - Stereophiles are good- and you’ll find curves showing how high frequency sound diminishes at 10/15 degrees off centre, and goes down from there. It’s in the nature of the all drivers,other than the rare omnidirectional efforts. So if you don’t toe-in, you’re missing high frequency sounds that are part of the music. People do adjust for taste; like food preferences. Speaker manufacturers primary frequency specification is for measurements made directly in front of the speaker. There’s the facts. Cheers

ptss

Regarding the wall behind the speakers. Regardless of the type of speakers the wall is a reflective surface. There will be a time delay between sound from the speakers and that from the wall. This is Distortion.

It’s really only distortion if the speaker system was designed to play in an anechoic chamber.

If you want refined,delicate,nuanced sound,at all volumes, this distortion Must be avoided.

How would you do that, short of an anechoic chamber? Or using headphones?

... keep speakers away from that wall. I like a minimum of 40 inches

Even at 40 inches, you’re still getting reflections off that back wall. I think the more important question is: How did the speaker designer intend for the speakers to be placed? Typically, dipoles are intended to be used away from the back wall. But many other speakers are designed for closer placement to the wall.

“Speaker manufacturers primary frequency specification is for measurements made directly in front of the speaker”

True statement but they also take those measurements in mono and only one speaker. They don’t and can’t consider more subjective elements like soundstaging and image specificity in any measurements. Optimum speaker placement is where all these elements are optimized as a whole and not one at a time.