Highest frequency that cannot be "localized" by the ear?

[theaudiohobby]

...between "omni-directional" and "non-directional"...subs should be considered the latter...anywho, around 150Hz may be the number...

Addressing other points of this thread:

FWIW, I tend to agree with Mssrs. Greene and RGA, with regard to placement of a sub...Since the sub's output is a mono summing of the lo-freqs, placement somewhere within the radiation field of the L&R speakers makes the most sense...preferably smack dab in the middle using one OR preferably two subs, one for each channel.

My rationale is: why would one want to perceive the lowest fundamentals from somewhere in the distance and the directional cues of their upper-harmonics from the appropriate loudspeaker?...I think it tends to obscure definition.

If I may chime in, implicit in the suggestion of placing subwoofers in the middle of two main speakers is that sub-80Hz frequencies may be directional. If As Tom Nousaine found, sub-80Hz are non-directional, placement of the sub for the smoothest bass response will not necessarily be between main speakers. I was much surprised when I first realised that the subwoofer integrated more seamlessly with the main speakers, when placed closer to to the right speaker, it is almost directly behind the right speaker, this location closer to the right corner of the room. And as mentioned elsewhere in this thread the crossover frequency for both the subwoofer and the main speakers 63Hz. This experience correlates more closely with Tom Nousaine's observations about subwoofer placement rather than generally accepted audiophile wisdom.

As for the multiple subwoofers approach, it probably accrues benefits not from even bass loading but from the extra clarity that may be available when the bass frequencies from discrete channels are routed to discrete devices instead of summation into a single device. Ofcourse this assumes that the subwoofers in question are seamlessly integrated with the main speakers.