I agree more with SunburnedCactus on this one. Death metal can be really complex and technical, it's not all simple and "neanderthal" like. Opeth do have a death metal influence, but as i said they are prog death so they play more complex stuff. Yes, there is some black metal there but death is a bigger influence imo.
Fair enough, i only have Leviathan at the moment but from what i've heard i'd say Mastodon were prog with a Southern Rock, almost stoner influence thrown in, what with the giant riffs etc.