M-Audio AV42 Guide D'utilisation page 5

Height
Position the monitors so that they are parallel to the ground and that your
ears are between the midpoint of the woofer and the tweeter.
Orientation
If possible, place the monitors in their upright position; avoid laying them
on their sides. The woofer and tweeter are vertically aligned and will
provide the best performance when the monitors are upright. Placing
them on their sides may create strong reflections from nearby surfaces
resulting in spectrally unbalanced sound.
If your monitors must be oriented horizontally due to space or line-of-
sight considerations, place them so that their tweeters are on the "inside"
to improve imaging.
Tweeters
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