Ah, this is an important issue.
Change obviously has no inherent value. Having discovered the proper means of communicating a certain aspect of reality, why would one ever be compelled to change this means? If one examines the art of all previous "Traditional" societies, be they the of the European middle-ages, the Viking societies or the primoridal Celtic civilizations, what we find is an almost completely fixed means of artistic and religious expression. This is because these peoples understood that only so many things (those which are most real) are worth expressing, and that there is always a contextually appropriate means of doing this, tailored to the needs of their race, in the most full and broad sense of that word.
This is why I become so annoyed by people who think that "metal needs to change and grow" in any sort of formal sense. Metal does not need to establish new techniques. It only needs to master the ones it has with the goal of finding the most pure expression of the reality it symbolizes. In other words, metal needs to understand what it is, not transform into something else.
In terms of extremely consistent bands, Angelcorpse is a good example, though I thought that their most recent album was somewhat of a less-pure expression of the "Angelcorpse Essence". I've recently been exploring Havohej (never liked them in my earlier youth), and it seems to me that they are extremely consistent about what they are fundamentally communicating, even if they occasionally employ some variation in form to achieve this. Thoughts on this band specifically, I would appreciate.