Mastadon are awsome, real sludgey (have i just made up a new genre?) kind of sound, they are really technical as well, not sure how well they will be openin for tool, totally different sound and not what every tool fan may find that accesible. Check out thier new album its amazing. They played reading on Sunday morning, but in my opinion the sound was terrible, ive seen them once before live before that, when they were touring thier last album, and it was excellent. they are well worth a listen before you see them live IMO