In a chapter from his biography of Mozart called "Fearful Symmetries," Maynard Solomon meditates on the peculiar qualities of beauty in the Mozart style. He considers the centennial of Mozart's death in the year 1891, and imagines the challenge faced by Brahms: "how to pay homage to Mozart without surrendering one's own individuality."
The Brahms solution was encoded in the Brahms Clarinet Quintet; an ensemble which entered the public consciousness with the Mozart's Clarinet Quintet in A, K. 581.
Listen with closest possible attention to the opening four notes of the Mozart Quintet. The tune moves down through a triad by skip then step to land on the first scale degree. This tone is harmonized with the relative minor; which delays the arrival of the expected tonic harmony until the chord that introduces the solo clarinet in the seventh measure.
The second movement of the Brahms Clarinet Quintet echos the intervals of the first three notes of the Mozart but then "block[s] the theme, refusing to allow it to continue, let allow to come to rest," observed Solomon.
[Thomas Friedli & Quartet Sine Nomine]
The restlessness is also rhythmic, with deep-scale syncopation and patterns with two-against-three creating gentle frictions. "In the end Brahms knew," wrote Solomon, "we cannot reach Mozart, we can only hope to come nearer to him."