Side wall placement is generally a bad idea outside of a quad sub configuration. In rectangular rooms, the width is obviously shorter than the length. You have two prominent room modes, basically frequencies the room amplifies, based on 1/4th of the length of the wavelength. In an 11' wide room, the frequency amplified by the side wall is going to be around 53hz. At the front/back wall, around 37hz if your room is 15x11 feet. Placing it in the corners will equally amplify both room modes, lifting the entire bass region up, improving the efficiency of your sub's by about 6dB, and providing the smoothest response. Seating distance is very important too. At about the halfway point from both the side and rear walls, the frequencies at the room modes will cancel out, since the reflected sound is out of phase with the direct sound. While this makes sitting directly in the middle of the room a bad idea, this cancellation can be used to your advantage. At 38% distance from either the front or rear wall, and 38% from the side walls, you're at a point where the reflected and direct sound just starts to meet, which helps tame the bump in response at the room resonance. Since you have sub's on either side of the front wall, this will be less of an issue as far as the room resonance along the width of the room goes. If you're able to move your couch, I'd place it at either 6ft or 9ft from the screen for optimal response.
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