Thanks for more insights and great info.

I made a mistake here where I tested M2v4 not M3. A typo. Sorry for that.

I guess that the floorstanding speakers have to deal with complex issues bringing by multiple drivers sharing the same spectrum of frequency. Bookshelf doesn't have that issue to deal with so the imaging could be better.

Let's say two tweeters produce the same note at the same time. However there are no identical tweeters in the world even they are manufactured by the same factory. The supposed same sound coming from multiple speakers actually are slightly different. When they are combined into the sound we hear, slightly different frequencies of the same note will make the sound less cleaner. That maybe the reason why imaging of even a small box like M2 with only two drivers could be so good.