Acoustic advice re: building a room (in wall or on wall) treatments

Look into the wall construction products made by ASC. I have heard a room constructed with them, and the structure of the room---especially the walls---was the least resonant I have ever heard, by a considerable margin. Rapped with one’s knuckles, the sound produced was a mere "click", not the normal resonant "tonnnk". The walls were like rock!

The ASC system begins by isolating the first layer of sheet rock from the studs it is normally secured to. A layer of WallDamp is then applied to that layer of sheet rock, and another layer of sr is then put in place. The WallDamp material is a constrained layer adhesive, and put between two resonant surfaces like 4’ x 8’ pieces of sheet rock (which left undamped vibrate/resonate like a drumhead) it creates a remarkably non-resonant structure. It can also be used in the construction of double-layer shelves for equipment racks, and in double-walled loudspeaker enclosures.

Having solid, sound proof walls is great. 

In terms of controlling the resonant character of a room though, I think that for most of us panels are a great way to go thanks to the flexibility and ability to grow over time.

I've been in the Magico demo room, and it's great, but it was built by expert acousticians. Unless I had that ability of no-hold-barred acoustic design I'd probably go with a mix of double wall construction as @bdp24 mentions, as well as after build panels.

Also, don't forget the ceiling, especially between the listener and the speakers.

I hope your room comes out well. I sure like the dimensions.

Building an optimal audio room is not a simple undertaking. Regarding your question:

• Rockwool will help isolate the room from outside sounds and vice-versa. I recommend it. There are other methods used to increase isolation, mostly various methods of wall construction. (A caution: the more successful the isolation is, the stronger the bass nodes and peaks might be.)
• Acoustic treatments (panels, bass traps) in the room will control reflection, absorption, diffusion of sounds within the room. They're how you reduce slap echo, first reflections, and bass boom.

One does not replace the other.

I suggest trying to read more about all this before finalizing your plans.

GIK and ASC will provide treatment plans for a small fee. ASC also will consult on room construction.

Have fun!