Didn't this issue come up in a series of posts a short while ago? One of the posters cited the Harmon web site. Their expert advice based on psychoacoustic research differs from that given in these posts. Given that LF has little or no directional information, a high quality sub well placed is usually sufficient except for very large spaces. Well placed is often in a corner. You might Goggle harmon, where you will find a series of technical papers dealing with various aspects of audio, including LF reproduction.
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