Thanks for the input.
Done 3" spacing with other horn tweeters and not noticed this in home.
And the distances in some PA applications tend to minimise this if I remember correctly. Noticed that the Jbl VRX932LAP has 3 tweeters in 13in height.
http://jblpro.com/products/portablesound/vrx/vrx932lap.html
Apparently horns do better than cones (or domes) in this domain.
Good you bought it up. Important to look for this my experiments.
Thanks
Barry