Also don't make the mistake that the fundamental is the only frequency to be concerned with. A low B is 31 Hz but most speakers can't reproduce that very well so it's mainly the harmonics (e.g. at 62 Hz and 124 Hz) you are hearing, which tricks your brain into hearing the fundamental when it's not there.
Anyhow, I think the main reason for wanting an octaver that tracks low is not to try and produce subs that you would struggle to amplify, but merely to stop it glitching when you do have to play a low note. Anything below regular open A sounds far too muddy with octave down, they're more suited to adding low end to higher register playing, which is something you can to an extent cop on an 8/12 string.