If you are going for sound quality, I would go with whatever has the biggest box. If the in-walls have a bigger box for the speakers, it would probably sound better than the smaller box on-wall speakers.

I have both the on-wall M22 and the bookshelf M22. The on-walls sound good, just not as good as the bookshelf because of the bigger box size.

The in/on-wall has a bigger box than the on-wall, but the on-wall is much, much easier to install.

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