Sounds like you are built into a bit of a slope or hillside if I’m envisioning that correctly being a walkout.
That’s a great ceiling height to work with. Very nice. I’m assuming your floor is slab concrete…are you building up a subfloor?
Which orientation are you using? L R channel facing the interior wall or the opposite?
I guess what I’m wondering is what were your considerations with the concrete in terms of how it will sound? Concrete gets a bit of a poor reputation overall in terms of sound quality and yet it has redeeming qualities and some great benefits too. Zero criticism at all , I just simply don’t know what to think when it comes to concrete and sound reproduction. Or is it all moot as you’ve built a purpose built room with in the concrete with its own surfaces for the sound to interact with? I guess it’s more about the bass and the concrete’s relationship…
