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.
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.