Regarding Welfarist Utilitarianism, there are no authorities on it. It's a rather loose term and more of a movement, although several ethicists devote their careers to hashing out their own perceptions on it. It isn't like Hedonic or Preference Utilitarianism, which are more or less tied to Jeremy Bentham and John Stuart Mill, respectively.
By contrast, however, Welfarism in general is a rather niche field and can refer to a wide variety of perspectives. It's currently more of a descriptive term than a prescriptive term. If that ever changes, I will probably need to come up with a new word to describe myself.
I consider the "authority" on Welfarism to be the evolutionary process itself, which imbues our DNA with a tendency towards a specific naturalistic teleology. I think a more accurate description might be something like "Survivalism," but that word has completely different connotations and health is generally a good indicator of how well one is surviving, anyway.
In that sense, the "authorities" would probably be doctors. They're the ones who debate on what "health" means, both physiological and psychological health. Another "authority" might be evolutionary biologists, who might one day refute the idea that evolution naturally selects for the survival of organisms, but I don't really expect this to happen. In both cases, the "authority" comes from valid expertise on the subject which anyone can learn and verify for themselves.
Outside of that, I do have a high reverence for logic, but logic itself is a methodology and not a set of beliefs. To perform logic well one must try to remain objective and be aware of the limitations of logic. For that reason, I do take some bits and pieces from Stoicism and Skepticism, but I don't necessarily consider myself a Stoic or a skeptic.
‡ Looking at you, Mr. Dawkins.
