Hi bw,
Given your listening distance, you'd have quite a bit of flexibility. I'd estimate you could have the tweeters as much as 18 to 24 inches higher and still be within the very tight "listening window" of the M80. The off-axis frequency response of the M80s remains very consistent to as much as 30 degrees off-axis (left/right and up and down) at a distance of 2 meters (about 6 feet), with only a little attenuation of the very highest frequencies even at 30 degrees.
You can see here the NRC anechoic measurements of the older v2 M80
http://www.soundstagenetwork.com/measurements/axiom_m80v2/ The new tweeters of the Axiom v3 series is even more consistent off-axis than the v2 series. The curve posted on the M80 v3 spec site is a composite of listening window measurements of the M80v3 in Axiom's own anechoic chamber, which duplicates the NRC chamber. The curve is significantly more linear than the older listening window curve on the Soundstage site.
http://axiomaudio.com/gallery_disp.html?image=M80_freq.gif&title=M80Regards,
Alan
