This is open to much debate. I've read anywhere from 70 to 120 Hz. My own experience is that it is closer to 110 Hz. But there's a catch. Often, the localizing cues we perceive when a subwoofer or speaker plays these notes isn't the frequency itself, but the harmonics, vibrations, interactions with room, etc. Your senses are pretty clever, and will hear the impacts of the released energy near the subwoofer, and hence direct your attention to that area. Play a 20 Hz tone sometime through your sub (or lowest it's safely capable of)...often your senses will direct you to the source...not just hearing, but everything.

With an A/V receiver, or subwoofer plate amp, look for crossovers or LFE cutoffs with steep acoustic slopes. 24 dB/octave. Many are still 12 dB/octave. No good IMO. Even 24 dB/octave still plays a lot of information above the so called "cutoff" frequency. Sometimes we're hearing these higher frequencies and mistaking it as 90 Hz being directional.

Often overlooked is the simple factor of the size of the room, your postion in the room and your position relative the subwoofer. If you're sitting near a mode or node, response will be affected quite a bit when moving even a few inches. The thing about non-directional frequencies is the size of the wavelengths. 90 Hz is 12 feet. If you're +/- 6 feet of axis, you can expect some major discrepencies between what you hear in your left and right ears. 80 Hz is almost 14 feet. 70 Hz almost 16 feet.

I guess my point here after all this rambling is that what's non-directional in your setup, might be easily localized in another room. Experiment.