Christian Friedrich Daniel Schubart.

My mistake -- it is he who wrote the poem "Die Forelle,"
not Franz Grillparzer
(who did indeed write Schubert's epitaph, though).

When I have enough time,
a future post to this thread
will submit something in the way of an English translation
so that everyone who doesn't have German
can share my disappointment (okay, disgust) in the text.