Fred, the only reason that I replied to that post was the comment that the "extra throw" would allow it to "reach lower". Yesterday I had run some of Keele's equations by hand(with the help of my pocket calculator). The results I got were in close agreement with your numbers and I didn't see a good reason to discuss them further. Briefly, using Vas and Qts I got an "ideal" enclosure volume of 1.55 cu ft., using Fs and Qts a -3dB frequency of 25.9Hz and again using Fs and Qts a 31.6Hz enclosure tune.

So, the point, as I mentioned above, is simply that the Xmax isn't a controlling factor in the frequency of the extension, but rather, along with other factors(primarily the effective diameter of the woofer cone)affects maximum volume capability.
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Enjoy the music, not the equipment.