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I'm not sure that time coherence is such a big deal in speakers, and of course JH has now released Freqphase.
 
 
 

Most loudspeakers are not time-coherent, and frankly I don't think this is very important in terms of the loudspeaker's perceived sound quality. At mid/high frequencies we are relatively insensitive to phase/group delay caused by the cross-over, or typical driver offsets.

Of the 350 or so loudspeakers I have measured, there is no correlation between whether or not they are time-coherent and whether or not they are recommended by a Stereophile reviewer. However, I feel that if other factors have been optimized—on-axis response, off-axis dispersion, absence of resonance-related problems, and good linearity—like a little bit of chicken soup, time coherence (hence minimal acoustic phase error) cannot hurt.

 
 
from: http://www.whatsbestforum.com/showthread.php?865-Audio-Science-in-the-Service-of-Art&p=8475&viewfull=1#post8475