Assuming the speakers are the Sonus Fabers listed in your system profile, you do not have to sit at the tweeter axis. Sitting slightly lower than the tweeter will not substantially change the treble balance. Stereophile reviewed the loudspeaker and performed measurements regarding dispersion characteristics. The review did note that loudspeakers where particularly sensitive to setup.
Tweeter height...align with ears?
I just purchased some new monitor speakers and stands. These stands put the tweeters about 3-4" above my ears when I am in my listening chair. I am just curious how important it is for the ears to be vertically aligned with the tweeters?