Hi, Jeannette--
i'm a little late joining this conversation, but can't help myself.
You've excellent suggestions so far. Still, you might want to listen
the 1st mvt. of Brahm's second symphony and the third mvt. of his
third symphony. None are fugues. The first example has canonical
entrances of the well known "Lullaby" theme. The second, while not
using imitative polyphony at all, is full of independent voices that
are well defined. Brahms was a fantastic orchestrator defended by
Rimsky-Korsakoff, himself. He merely said that we don't run to
Brahm's scores because he had no interest in inventing new
combinations of sound. Still, his sense of mixing, balance, and
definition were infallible.
Some other pieces to consider: Bartok's Concerto for Orchestra
(finale), and Britten's Young Person's Guide to the Orchestra (you can
just search for the fugue). Getting away from "B" composers, try Nino
Rota's Concerto for Trombone and Orchestra (1st mvt.) and the second
mvt. of Stravinsky's Symphony of Psalms (which opens with a canon in
the winds.)
i hope that helps!
Tom