Complex sentence: In German sources can be found the name Ferdinand Haase (1788--1851), who worked in Russia in 19th century, but he simply wrote the second Marsch des Leib-Garde Preobraschenski Regiments, not the famous one.
Split into simpler sentences: In German sources the name Ferdinand Haase (1788--1851) is mentioned. Haase indeed worked in Russia in the 19th century, but he wrote the second "Marsch des Leib-Garde Preobraschenski Regiments", not the famous one.