That may be why you don't hear a difference between QS10 and M5. Ears are less sensitive to noises from the back and even directly from the side. I have my QS10s forward. When I replace them with M5OW, the sound is more direct and the mids and highs are slightly more prominent.
. Even though What Mojo says is correct … side/rear surrounds should be 90 deg and/or behind you. That you don’t hear the difference is, IMHO, placement, room, material played, and ear dependent … the QS10 surrounds should be more diffuse… I.e. harder to detect the exact direction the sound came from. if there is nothing for the sound to bounce off of then you might only hear the sound coming directly at you.
