Which Grados have you tried? And of those, what pads were you using? Comfies or bowls? (In my experience, bowls really open up the sound on Grados, providing more natural soundstage and wonderful sounding bass response and better defined mids).
All Sennheiser I've tried make metal sound muted, dull, and blurry (compared to Grados anyway). What types of metal are you listening to?
Open headphones for metal music = Grados
Strong mids = Grados
Good bass, but not too strong = Grados
Really, I'm surprised you didn't like Grados for metal. Now I love metal, and in my experience, too large a soundstage just sounds weird on metal. If you're sure you like a big soundstage for metal, then perhaps you're right to look for something other than Grado, but I think the signature Grado immediacy is a serious boon to metal. Not only that, but Grados (I'm thinking SR80i & SR125i) have great punchy bass that works great with metal, and the highs are forward and exciting. Every frequency presentation of Grados are made for metal, including the small soundstage.
Maybe give Grado another try? Try SR80i or SR125i with L-Cush pads (bowls)! SR80i best value in Grado line imo, and SR125i is the best sounding Grado in the Prestige Series imo (most would say SR225i though...I never got that though).