When he returned, he studied theology and philosophy at the universities of Paderborn, Frankfurt and Bonn, and at the Priesterseminar Aachen [de].

His dissertation dealt with the sacred choral music by Johannes Mangon who had worked at the Aachen Cathedral in the 16th century.

After Rehmann's sudden death in 1963, Bishop Johannes Pohlschneider [de] appointed him as director of the Aachener Domchor and Domkapellmeister.

He improved the choir's quality, including the boys from the Domsingschule, leading them to international recognition by tours in Germany, Belgium, the Netherlands, Ireland, France, Italy, Poland, Czechoslovakia, Switzerland, Austria, England, Spain and Israel.

Programs included masses, oratorios and Passions, also in recordings, and radio and television broadcasts of services from the cathedral.