If those Kappa Perfect ported cabinets were built to spec, including the high density acoustic absorption materials and 13 ply birch, you should be able to use them to at least 95-100 Hz if a 4th order xover is used, and have perfect fidelity. The port resonance of this design is of such narrow bandwidth and of low enough relative level that it will not be audible even though it's only a fraction of an octave above the highest xover frequency I specified above. Now, to cross at 100Hz would require twin subs to be placed on each side of the room, corresponding to a relatively short distance from each main, if one wants transparent/perfect integration for music purposes, especially things like cello, acoustic bass, etc.. For things like movies, however, it's not nearly as important.
-Chris