I love classical music. When I listen to classical, I tend to listen to:
Sibelius, whose austere majesty, though non-ambient, certainly can find a resonance with ambient music
Satie, a father of ambient music
Milhaud, someone who appreciated sound as sound
Copland, arguably not "ambient" at all, and yet someone willing to put aside the formalism of his time to find a way to use sound to connect with an audience--to me a very ambient idea, in that our music is in some ways a different kind of individualized folk music.
early chants and church vocal pieces, which are ambient in design and execution
Even composers who might not be "ambient" even by analogy, such as Tchaikovsky, make densely textured music which speaks in broad instrumental passages--a reward for patience that any ambient fan completely understands.
Dark ambient fans appreciate how Schoenberg's work is essentially kindred to dark ambient material made under its shadow.
Among the modernists, Philip Glass' minimalism resonates with ambient minimalism (which is, actually, a by-product of the application of the minimalist theories to our more "folk" idiom). Golijov's music is "narrative" in its sound, and yet part of the joy of hearing it is that it is a celebration of sound itself.
Lately I think we must "save" classical music from a generation no longer attuned to it--and in so doing, we will save and extend the ambient listening base as well. If one bought 1 classical album for every 4 ambient ones, then perhaps the world would be a more classical--and ambient--place.