I cannot quite help with Tieck/Wackenroder, only that I also recall that at least some of the music that inspired them supposedly was by Reichardt and others who are almost forgotten (except as mere names) today.
We do have lots of material from E.T.A. Hoffmann who is in the same generation but writing somewhat later in the 1800s and 1810s. Unlike the others mentioned he was an accomplished musician (although he had to work in law because he never secured a musical position for a sufficient time to make a living). The central text is the the famous extended review of Beethoven's 5th (Beethovens Instrumentalmusik).
Hoffmann definitely helped to establish the "classical triad" (or tetrad if Gluck is included) and his points of reference are usually late Mozart and Haydn as well as early and middle Beethoven (and he also knew music by Palestrina, Bach and Handel). (So unlike maybe Wackenroder he *does* refer to music we still consider of the highest quality.) The interesting point in Hoffmann is also that he basically inverses Kant's dismissal while making use of some Kantian categories. He agrees with Tieck etc. that music is not about anything in particular (he makes fun of battle and storm symphonies and the like) but that it somehow expresses "infinity" which is linked to the aesthetic category of the "sublime" for Kant and which cannot be given in words or in common experience. In that review he also uses words that remind one of "occult mysteries" (italics added by me):
"When music is discussed as an independent art, should it not be solely instrumental
music that is intended, music that scorns every aid from and mixing with any other
art (poetry), music that only expresses the distinctive and unique essence of this art?
It is the most romantic of all arts, and we could almost say the only truly romantic one
because its only subject is the infinite
. Just as Orpheus’ lyre opened the gates of the
underworld, music unlocks for mankind an unknown realm—a world with nothing
in common with the surrounding outer world of the senses. Here we abandon definite
feelings and surrender to an inexpressible longing."
"Thus Beethoven’s instrumental music opens to us the realm of the monstrous and
. Glowing rays shoot through the deep night of this realm, and we sense
giant shadows surging to and fro, closing in on us until they destroy us, but not the pain
of unending longing
in which every desire that has risen quickly in joyful tones sinks
and expires. Only with this pain of love, hope, joy—which consumes but does not
destroy, which would burst asunder our breasts with a mightily impassioned chord—we
live on, enchanted seers of the ghostly world
! ["entzückte Geisterseher" could also better be translated as "ecstatic visionaries", Hoffmann certainly does not mean anything spooky but seems to compare he experience of music to spiritual, visionary ecstasy]
"Thus he [Beethoven] is a purely romantic composer, and if he has had less success with vocal music, is this because vocal music excludes the character of indefinite longing and represents the emotions, which come
from the realm of the infinite, only by the definite affects of words