Way to stick with it and work out the root cause. With that out of the way, how does the whole thing sound?
My basement still isn't 100% done (waiting on doors) but the setup sounds awesome! I was able to watch some of Blade Runner 2049 (in Atmos) turned way up yesterday and was blown away by the level of detail and immersion.
I'm also finding that my in-wall M5's sound much better now that the tv has been wall mounted (it was previously sitting on feet, about 6" forward of the wall). At no point was it blocking line of sight between the M5s and MLP, but it was clearly interfering with the sound field in some way.