Reading the wonderfully clear & accessible critique of string theory, The Trouble with Physics, by Lee Smolin. An education in scientific method.
I'm in the 1st chapter. Discussing how scientists seek to explain diverse phenomena through unification in a comprehensive theory.
What would a "theory" of American poetry look like? It would have to be grounded in a particular idea of the nature of poetry in general, and in a good grasp of the literary history.
Probably some such set of principles (defined or implicit) is part of the equipment of every good critic.
The idea would be to unify one's judgement of the various approaches and styles based on such a "theory" (or set of principles) - & thus offer a persuasive interpretation of the jostling tribes.