I would strongly suggest getting a pair of rigid stands for the M22's. My pair are on 20 inch fillable stands.
There's at least two problems I can think of with putting your speakers on a shelf. The first is that the shelf might not be rigid enough, and that will hurt you in terms of sound quality. The second is that a shelf position is rarely ideal for speaker placement. Even though the M22's are called "bookshelf" speakers, they need a little breathing room. I have my speakers 18 inches from the front wall (measured from the face of the speaker).
Putting your speakers on stands is a nice way to display them, too. Axioms look great so show them off!
