666 is a reference to Hell.
In GH3 solo career, the last venue is Hell. Go one venue before that (-1) and it's the Japan venue.
Looking at the songs in that tier, we have all of them in Rock Band already except for one:
"Knights Of Cydonia" - Muse
I'll be honest. I got a Muse album at a garage sale for $1. D@mn good album.
Now I'm thinking that it is singles and that "Knights Of Cydonia" will be one of them.
While obviously I'd personally love a Muse pack of 6+ songs, having at least one other song of theirs to play in Rock Band would be so great.
DAMMIT RBR! YOU'VE BROUGHT BACK HOPE! I HATE YOU!
I actually want Madness.
Bring on the Muse!
I want Supermassive Black Hole