At what freq. does sound become non-directional?

It's not the directionality of the soundwave (which will vary with dimensions of walls, baffles, distances to boundaries, etc. ad nauseum!), but the issues re proper bass-loading in your room. Best results are usually obtained with multiple woofer locations, so that you have some chance of balancing and averaging out room nodes.
The human ear, however, has trouble discerning direction below 100-150Hz, right? OTOH there may be enough harmonic overtones and/or distortion artifacts, or even noise contributants that are indeed very discernable due to their higher frequencies. It's not that low bass is or isn't directional, it's that you can't perceive it as such without
higher frequency cues.

You really will have to use your ears alot on this one. THX standard is 80hz and most crossovers range from 40hz to low 100hz+. If your room is 70feet wide, you'll hear stereo bass at any frequency, since the length of even deepest soundwaves is less than all the boundaries. You'll have to compensate for the room gain and watch the overlap throughout the crossover region to get as smooth a response possible

I definitely agree with the first reply it's definitely directional you can hear where the bass is coming from.

I can turn off my main amps and sit there listening to my two Entec woofers imaging all over the place below the 100hz crossover.....Other than moogs and organs not much information below 50hz I want to listen to, but it adds space to the presentation and surprising how much venue information is extremely low frequency as size of the stage grows about 25% when woofers are switched in over the main speakers running full range (Rockport Syzygys which are good into the thirties).....

The ear becomes less sensitive to localization and directionality clues at the lower frequencies, say from 20-60Hz. In a home listening environment, however, where the sound is enclosed in a relatively small space (compared to the outdoors), and the listener is fairly close to the speakers, most people can sense localization clues from the subwoofer. One of the best ways to avoid or reduce this problem is to use two subwoofers at opposite sides of the listening room, preferably with the subs placed in the room corners. The other argument for having two subs for music applications is that the left and right channels often have differing phase and amplitude characteristics which are lost when a summed L+R signal is sent to a single subwoofer.

Your situation is a bit more complicated because you are using monitors rather than full-range speakers. One of the toughest problems is to get the subwoofer to properly integrate with the main speakers. To achieve really good integration requires main speakers that are close to full range (fairly flat down to at least 40Hz), so the frequency (crossover) overlap between the main speakers and the sub is minimal (much of this is based on filter theory, which is too technical for this discussion). The higher the frequencies that must be covered by the subwoofer, the greater the likelihood that you will be able to determine the location of the subwoofer.

If you go to the Vandersteen Audio web site (www.vandersteen.com), there is an interview (done by Widescreen Review) with Richard Vandersteen, who provides some technical yet understandable information about subwoofers, and the rationale for having two of them.