however I have learned that instruments themselves should not come from beyond the speakers which is what you would expect if you think about it. I use to occasionally hear some instruments on certain recordings come from beyond the speakers.
I will have to completely disagree with you on this.
A symphony orchestra is set up with violins stage far right, double basses are far left. The main mics are, usually, setup in a Decca Tree config (which is used almost exclusively for orchestral recordings), the 2 mics used to capture stereo, are omnidirectional, and they are usually fairly close to each other. So, they are definitely capturing information from well beyond their outer 'edges'. There are also "outrigger" mics to capture a bit more ambience.
On a decent classical recording, there is plenty of information from beyond the outer edges of the mics, and if the speakers (and room) are relatively good, those violins stage right, and basses stage left, will come from beyond the outer edges of the speakers. And it's not a gimmick, it's on the recording.
Maybe if the information that was coming from past the outside edges of the speakers was of instruments that aren't supposed to be to the far left or far right, like clarinets or some other instrument from the closer to the center of the orchestra. If a clarinet seemed like it was coming from beyond the edges of my speakers, that I could see blaming on artifacts of a poor room. But the fact that the instruments that always seem like they are coming from past the edges of my speakers, are only those at stage right or left, seems like a bit too much of a coincidence.
I've got great diffusion behind the speakers, absorption on the side wall and ceiling.
Not to mention, I've heard a large pair of Wilson Alexandria speakers in a room so large and well treated, that I would be surprised if the side walls came into play at all. And again, tons of very natural sounding orchestral instruments coming from past the outer edges of the speakers.