Rotating the tweeter won't do anything as the tweeters are omnidirectionl (sending sound in all directions).
Picture a "barrier" between each tweeter and woofer. If you put a speaker like the M5 on its side, those barriers will prevent the sound from going full left to right in your room as the tweeter will be "blocked" by sound energy coming from the 5.25" woofer, and the 5.25" woofer is somewhat "trapped" between the tweeter (yes, it would overpower the tweeter, which is another problem) and the 6.5" woofer. Now the 6.5" woofer is like the tweeter. One side completely open and free, and the other side boxed in by the 5.25". Again, these are all left-to-right limitations when the speaker is on its side.
Put that same M5 in a normal vertical position and those "barriers" are no longer left to right stopping the listeners across the room from hearing the same quality speaker, but now they are top to bottom, and no limits left to right. That means that in theory, a person sitting to the left or right (within reason) of the middle listening seat will hear the same about of tweeter, the same amount of 5.25" and the same amount of 6.5" as everyone else. The sound is free to go left and right without (as much) cancellation as if done on its side.
There are ways to combat this like done in the VP180, but there are a number of people that say that it is best to put the tweeter above (or below I suppose) the woofers in a center channel, just like any bookshelf or tower speaker has them. That is why the VP160 has the tweeter above the woofers in its configuration.
