I don't know about the in-wall, but I reported a couple of years ago about the large amount of comb-filtering I discovered with the On-wall M22's. This may be a problem with all on wall or in wall speakers. I don't know. It was enough to turn me off from them, though. The boxy M22's did not have this same problem.
I would play a constant pitch sound at a constant volume, using one of the CD's I have with these kind of sound tracks on it. As I slowly walked from one side of the room to the other, holding a radio shack sound meter (pointed at the speakers), my ears and the meter would pick up the volume differences that would go up and down as I moved. They would stay at the same level with no movement. There were a lot of variances and only an inch or 2 apart from each other.
I've never heard anyone else prove this one way or the other.
