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.