When it comes to localization, vertical cues being anywhere from 1-2khz (it obviously varies on the person). If you can cross your tweeter low at a steep slope, then it is not overly important how close your tweeter is to the mid vertically. Of course, separation laterally can be damaging, but again, a lot of this can be worked on with EQ, by correcting SPL and frequency response anomalies.

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-Neil-