baja,
In fact, "niche" placing is not very good for ANY speakers, front ported, rear ported, or sealed. The majority of speakers (either bookself, despite the name, or floorstander) sound best when placed alone away from the surroundings as much as possible. If you have to place M22's or M2's on those shelves, I suggest you empty other shelves immediately next to and above/below the speakers as much as possible.
Horizontal placing of M2/M22 does not cause any physical problems. But horizontal treble dispersion would be more limited and become asymmetric from the right to left per speaker. It may adversely affect the soundstage rendering.
