""Placing speakers against the wall doesn't not affect the 3rd dimension""
Moving the speaker away from the front wall does TWO things. It effects the response of the speaker in the lower midrange/upper bass depending on the distance and it you move them far enough away the first reflection becomes an echo. What you are interpreting as the third dimension is that echo. The third dimension is the realization of the depth of the instruments, a marker of the size of the instrument and there location in space.
Echoes that you produce are distortion 😂😂🤣😅😏