Hi Philippe,
The off axis performance is rarely improved by the tweeter as tweeters in general tend to have good dispersion in the critical band. Improving the off axis of a tweeter design will normally be in the very high frequencies that are pushing the upper limits of our hearing (and the new tweeter has this improvement off axis due to the new shape of the face plate coming off the dome).
A good way to improve your off axis performance would be to go to a model like the M22 or M60 through M100. And even though these models will all give you a better off axis performance than you currently have, if you really want to have great off axis performance the way to go is one of the models in the LFR series. As Digital Trends commented in their professional review “the omni-directional LFR1100’s allowed us to move almost anywhere in the room and hear a balanced stereo sound … like those old portraits that stare at you no matter where you sit in the room”.
