Zimm is right, the center of the room is the worse place to sit, right in the null of a room. 38% from the front wall or back wall is ideal. My room is bigger than yours and I have my Q's hanging 7ft off the ground (bottom driver). The left surround is the only one with a wall behind it.

You could put the Q's on the back wall and adjust the distance, dB levels to compensate when calibrating.

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