RayLewis,
I currently have M3's in a entertainment center/bookshelf and have them sitting out as far as possible to give the rear port some breathing room and to reduce any reflections from the entertainment center itself.
I USED to have them on their sides. I was happy with them in that configuration, but noticed this weird null in the sound right outstide of my listening area. Not really a big deal, and they sounded great.
I decided just to try a different position and had them standing upright, and higher up on the entertainment center. Still sounded good, but that weird null was gone. Could be that it went away when I raised the speaker up, turned it upright, or both.
Bottom line is that it works, just try different positions if possible.
