(digging way back in the cobwebs of memory)
I built some speakers with dual 8-ohm servo subs in each speaker years ago, in which the internal cavities were designed as cylinders for the structural advantage described above. In addition to being cylinders, no further skeleton-izing of the internal cabinets/bracing was done (these were stacked mdf constructed cabinets) so aside from being insanely heavy they were probably as inert from sheer mass as could be built.
When designing the speaker cabinets, if I am remembering correctly, the only thing Danny mentioned when reviewing the plans was to make sure the length/dimensions of the internal tubes did not create a problematic standing wave situation within the frequency range the subs were needing to play. I vaguely remember my tube dimensions were the driver cutout diameter by about two feet. A stiffly built more traditional box of different dimensions would have been (at least on paper) more ideal, but my implementation seemed work work well enough. However, take that with a grain of salt as I didn't at the time have any way of objectively comparing the performance of the cylinder-based cabinets to a more common box design.
For what it is worth, having now owned front-firing sealed box versions using the parts in that kit, if I were to do it again I would go with a more traditional (and front-firing) box design rather than the cylinder. It would take less space in the room, less materials, and because I was often playing the subs fairly high up in frequency front firing them seemed to work better integrating them with my mains (when run in stereo pairs).
