I had poor luck deadening behind the Quads. I put home made diffusors behind them, which helped a lot. They are "skyline" designs (2D diffusors). That said, I still preferred them 5 feet out, and had the space to do it.
Optimally 4 to 5 ft to get the delayed response from the back wave sufficiently separate from the direct signal. If you deaden the wall behind the speaker you might get them a bit closer but I think you then kill the added ambient/ reflective sound and if you are going to do that you're better off getting some good box speakers.
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