Greenleaf expanded and upgraded the Conn plant, and converted distribution from mail-order to retail dealers.  By 1917 the assembly-line work force had increased to 550 employees who were turning out about 2500 instruments a month using a new hydraulic expansion process which Greenleaf introduced to the plant.  In 1917 Conn introduced the Pan American brand for its second-line instruments.  Conn founded the subsidiary Pan American Band Instrument Company in 1919 and later that year moved production of second-line instruments to the old Angledile Scale factory. In 1930 the Pan American company was absorbed by Conn, but the Pan American brand for Conns second-line instruments remained in use until 1955. Conn founded the Continental Music retail subsidiary in 1923.  At its height, the operation included a chain of over 30 music stores.  During the 1920s Conn owned the Elkhart Band Instrument Company (1923–27), the Leedy Company (1927–55), a manufacturer of percussion, and 49.9% of the stock of the retailer Selmer Company (1923–27).  Conn purchased the drum manufacturer Ludwig Drums, the instrument import/retail operation of the Carl Fischer Music company, and accordion manufacturer Soprani in 1929.  From 1940 to 1950 they owned the Haddorff Piano Company, and from 1941 to 1942 the Straube Piano Company.

Answer this question based on the article: Which company did Conn own the longest period of time, Haddorf Piano Company or Straube Piano Company?
Haddorff Piano Company