The best I can figure is that the 5/7 Stereo modes are automatically disabled if certain speakers are set to none. If the surrounds are not present, you have nothing by stereo (no "3 stereo"), which makes sense. Oddly, if you have no center, there is also nothing but stereo (no "4/5/6 Stereo"), which may be related to the treatment of the "no surround" condition. Also, this is only limited to the 5/7 Stereo mode -- PL II and NEO6 still work fine with no center channel.

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