Brahms isn't that far removed from our time. I can
think of three conductors to listen to to get an idea of
how his music was played in his days. Felix Weingartner
knew Brahms personally. In fact Brahms was pleased
by the conductor's interpretation of his 2nd symphony.
There are also Bruno Walter and Sir Adrian Boult's
recordings of Brahms' symphonies for modern day
conductors to look to as the link to performance
of classical music in the 19th century.