With tweeters say X" ctc, the combing that occurs at listening distance will be a sort of ripple in the reponse, with the troughs about X" apart.. now our ears are like 2" from top to bottom, so they will average out any variations within those 2", so maybe combing will not be audible once the tweeters are closer than about 2" ctc.

As I said, just a random thought.