Hi,
Putting any speaker on its side does change the dispersion characteristics and, to a degree, undoes some of the designer's careful calculations. Putting the speaker behind cabinet doors on a shelf will further affect the sound. If you must do that, try and keep the front baffle of the speaker as close to the edge of the shelf as possible, to avoid further tonal coloration caused by the shelf and surrounding cabinetry.
You can check yourself to what degree the sound will change. Put one M3ti outside the cabinet on a stand of some sort in its normal vertical orientation, and the other on its side inside the cabinet door on the shelf. Play a CD in mono, if you can, and rotate the balance control from one speaker to the other, adjusting the volume control to keep the sound level constant. You will notice a difference, but it may or may not be minor.
If you have an A/B speaker switch, you could wire one speaker to the A terminal and the other to the B terminal and switch between the two, keeping volume levels the same and listening for tonal differences.
The distance from your center channel should be okay for a reasonable degree of stereo separation.
Regards,
