Right. Everything flat reflects sound like a mirror. From the beginning when I had nothing, no rack no turntable just speakers, a little Kenwood integrated sitting between them, even then I started noticing. Anything at all in the center reflects and makes the image less solid. Even one record album, I found out by accident when someone came over and noticed. Did not know why, just noticed the change in sound. I heard it too. Only after he left I figured out it was one record left out, my weak attempt at decorating an otherwise empty room. So I put this record out like art. Ruined the sound stage. One record.
Another time I got a great big tube trap in trade. Which is how I know these are not what they're cracked up to be. But whatever. To each his own. One thing I tried, dead center right in front of the components. Sounded like Sinatra was somehow singing from inside the tube trap. That probably sounds bad. I don't mean muffled like actually inside. I mean like no tube trap but instead Sinatra where the tube trap is. So it was good. Not for what everyone thinks trapping bass. It was good for this. Except for the part about it being the ugliest most inconvenient thing ever, it might be there today. Instead of hanging in the shop over my table saw. Works absolutely great over the table saw!
A few inches applies to frequency response. Even a few inches relative to a large flat surface like a wall makes a very noticeable effect in response. So there's that. But the rack thing, here its something different. Here its arrival times. What you're trying to do is avoid sounds arriving too close in time to the main L/R signal. You also don't want sounds arriving too close in location to the sound stage, at least not unless they are delayed a lot in time.
We're talking 3 to 5 milliseconds. I'm going by memory, it could be 5 to 7 ms, but its something like that. Duke is the man on this. Anyway sound travels roughly 1 foot per millisecond. That's not precise, that's back of the envelope, which is good enough. First reflections within that time frame crush the image illusion effect. That's why everyone says put speakers at least 3 feet out from a wall. Makes the sound travel an extra 5 feet or so, pushes the reflection outside the 3 to 5 ms delay, sound stage is great.
Yeah, science!