I can see how an on-wall or in-wall center is no different than a free-standing one from a sound quality perspective because a center speaker does not lend to the illusion of stereo. There is no stereo image for a center. The left and right front channels though might be a different story. I don't think you can get a 3-D soundstage with on or in-walls but I could be wrong because I've never heard them. Maybe it's possible and if so, that's a secret that we should all know about. If so, why buy floor-standers and take up all that space?
I used to listen to music in PLII. Since I got the v4, I've ran into very few songs that sound better in PLII. The holographic soundstage is adversely affected in PLII. By holographic, I mean the soundstage is wide beyond the speaker boundaries, deep and tall, images are well-defined like light spectra through a prism and there is a sense of blank space between those images.
Not sure you can achieve what I described above with on-wall M5s.
