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With a baffle that size why use a singe 12"? 
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A single 12"?  I don't believe I mentioned what to use for bass augmentation, maybe I don't understand your question.  FWIW, I have been using a single 15" Augie (per speaker) for everything below 200 Hz in my system for quite some time with excellent results.  This of course is in my small space, YMMV.
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I meant a single 15. Whatever works!