Determined by his father before his early death to become his successor, he studied business administration at Cologne University – after returning from the front – which he completed with the thesis Über die Organisationsgrundlagen des deutschen Kulturtheaters [On the organisational foundations of the German cultured theatre].
He completed his "apprentice years" at the Bavarian State Opera in Munich under the direction of Rudolf Hartmann [de], Heinz Arnold [de] (directors), Georg Solti, Eugen Jochum, Ferenc Fricsay (conductors), Carl Orff (composer) and at the Bayreuth Festival under Wieland Wagner (director), Hans Knappertsbusch, Herbert von Karajan (conductors).
In addition to the established opera repertoire, his productions also included American first performances: in 1954, Honegger's Jeanne d'Arc au bûcher, in 1958, Orff's Die Kluge (title role Leontyne Price) and Carmina Burana (conductor Leopold Ludwig), (stage design Jean-Pierre Ponnelle), in 1959, Richard Strauss' Die Frau ohne Schatten, in 1960, Shostakovich's Ekaterina Ismailova.
In addition, guest performance engagements took him to the Mannheim National Theatre (Salome with Gloria Davy in the title role), to Stuttgart, Cologne, Lyon, to the Milan Scala (Mascagni's Cavalleria rusticana) and the Teatro Colón in Buenos Aires (Der Rosenkavalier with Sena Jurinac, Christa Ludwig, Walter Berry, conductor Erich Leinsdorf).
In 1976 in the Opernhaus Dortmund, Siegfried completed Wagner's Der Ring des Nibelungen, which had begun with Die Walküre, conducted by Wilhelm Schüchter.