Why are you interested in phase at 20 kHz? I don't think there's even a point going above 10 kHz. The lower kHz range might be interesting if there are crossovers and bass-mids for reasons mentioned above.
On the other hand, it might be another thing some people will point at and say: uww there's some phase shift, must sound bad! So maybe we're better off without that extra data.
Edited by xnor - 2/21/13 at 12:22pm