I don't think there have been any direct comparisons between the completely in-wall M22s and the bookshelf models, but the on-wall M22s don't have as much bass as the bookshelf models. I assume that's not the case with the in-walls, since they're deeper than the pure on-walls and therefore have more interior volume to work with. The in/on walls are a different matter. Confused yet?

I am the Doctor, and THIS... is my SPOON!