Aha! The most recent Am Civ class has cast a wonderful light upon the Teufelsdröckh chapter whose references to Wagner so mystified me earlier. German Romanticism as it connects to American Transcendentalism! There, my friends, is the key. Henry Adams liked neither one, particularly as the wife who committed suicide during that 20 years of silence was an American (Concord) transcendentalist who followed in her mother's, aunt's, sister's and brother's footsteps in deciding that a clock-work universe with an indifferent God wasn't worth living in.
German Romanticism -> Transcendentalism -> Existentialism?
That's the question. I believe a defensible argument could be made for their essential similarity and, who knows, maybe Henry Adams has already made one. I will continue to read and see.