ok, lets rethink this. Your ears pick up from about a 2" height, so anything less than 2" apart should not be able to be heard as a changing volume level, especially at 3m or so distance.

Logically, comb filtering in tweeters should manifest itself, not as a change in level as you move up and down, but as a general reduction in the average spl level of the very high frequencies as the peaks and troughs average out.

For us old guys, this is basically irrelevant anyway ;-))