Philadelphia Orchestra audiences are, by and large, a quiet, docile, respectful group. So no complaints were heard at the beginning of intermission (about a quarter-hour after Monday's concert had started in the Kennedy Center) because the Brahms Academic Festival Overture had been substituted for the Passacaglia, Opus 1, of Anton Webern.

It is, of course, a substitution of some significance -- as though one had ordered Szechuan-style beef in a Chinese restaurant and been served chow mein instead. Both works are Viennese and they were written less than 30 years apart, but one is comfortably old-fashioned and familiar while the other proclaims, in its own terms, that the 20th century has begun.

The reason for the substitution of works was a substition of conductors; instead of Klaus Tennstedt, William Smith, who has been the orchestra's associate conductor for a quarter-century and who is, fortunately, well-versed in Mahler's Seventh Symphony, the other scheduled work.

That is no small accomplishment; the Seventh is in many ways the wildest and most complex as well as the most joyful of Mahler's symphonies.

It was superbly played, of course; the rapport between conductor and orchestra could hardly have been better, the balances were fine (allowing the mandolin, for example, to cut clearly through the orchestral texture), and the spirit of the music, which changes mercurially from one minute to the next, was presented in all its variety, from dark mystery to pure playfulness. The quality of performance helps, no doubt, to explain why this orchestra's fans are unruffled by last-minute changes.