Tip of the Day: Speaker Placement & Early Reflections
Give your speakers plenty of space to breathe around and in front. Things like TVs, Shelves, Other Speakers, Coffee Tables and Walls create secondary sound sources that smear the presentation through early reflection and diffraction. Let the speaker be the only thing sending early cues to your ears.
That said, side walls can have definite presentation benefits - especially if you can give the secondary sound roughly about 15 milliseconds to travel - or a rough distance of 5 meters from speaker-to-reflecting-wall-to-ear.
Source: Forum Member - GranteedEV