Elias, a belated welcome. When sitting very close to the back wall it's unlikely that a very satisfactory back surround effect can be obtained with a 7.1 setup. There have been some who have reported that placing the 6th and 7th speakers on or near the floor along the back wall and firing up at the ceiling was reasonably effective. It may be advisable to stay with 5.1 and to mount the side surrounds about 6' up on the back wall and widely separated, possibly about 1' from the side walls.

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Enjoy the music, not the equipment.