Nick, what do you consider might be "wrong" about your recollection? The 17.6Hz number is in the 17-18Hz range you mentioned. Of course, if you measured the volume of your enclosure and it's significantly less than the 5.83 cu. ft. "ideal" for a maximally flat response curve, then the extension at -3dB would be less and there would be more of a peak in the response.

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Enjoy the music, not the equipment.