It's the consensus on this board that mounting any vertically-designed Axiom speaker sideways is not a good idea.
The tweeters are designed for good horizontal dispersion. If you place them on their sides, they'll disperse vertically instead. This will most likely adversely affect their most cherished qualities: detailed imaging and soundstage depth.
Another reason not to put them in your entertainment center is that the M22ti speakers are rear-ported, and thus need clear airspace behind them. Putting them in an enclosed space like the one you describe will adverseley affect their sound.
Hope this helps your decision.