My interest in the 12c would be to replace old automotive subwoofers that found new life in my home theater set up. Xmax is 13mm IIRC. That would be fine for me as a subwoofer but a lot of sub now have ~18mm xmax that is preferred by others.
danley spec'd the original lab12 for the lab horn project. it also found use as a ported sub in imax installs. xmax on it is conservative in the sense that there is much more 'usable max' or whatever you want to call it than it may appear.