In his Harvard dissertation, American musicologist Robert Levin focused on Mozart's incomplete works. He developed Mozart's sketch for the Amen fugue into a complete composition. German conductor and sacred music specialist Helmuth Rilling was so impressed that he asked Levin to revise the entire Requiem. The premiere took place in 1991 on the 200th anniversary of Mozart's death. If you're used to the Süssmayr version, Levin's Amen fugue at the end of the Lacrimosa and his revised Osanna fugues in the Sanctus and Benedictus are, to say the least, unconventional. Elsewhere, he acts more as a subtle restorer.