Your welcome lomi.
You should PM levi as he's done a number of rooms.
It seems you're most concerned with keeping sound in; I reckon the double wall will help. Hopefully you have a cement floor too, to keep your sub's bass from leaking too much.
If you're using the double wall to hide cables, see if you like a conduit (big-diameter pvc pipe) so you can pull them when you need to change them.
I used double glass windows, which are effective for keeping sound in, and out. My only leak is through the door, which I could block with upholstery from a leather seat-cover maker if I wanted to.
You still haven't addressed acoustics
inside the room, which are much more difficult. But neither have I
as I'm still breaking in gear and learning about acoustics.