According to Eugene Chadbourne he held a job in Van Alexander's orchestra when he was sixteen years old. See Van Alexander at Allmusic. But this is impossible, since Manne didn't even learn to play the drums until late 1938, when he was eighteen. After that he began to sit in on 52nd Street and then played for a few months aboard a cruise ship in early 1939, after he graduated from high school. Following further engagements at resorts in the Catskill Mountains and more sitting in here and there, he got his job with Bobby Byrne in 1940. George T. Simon says that Alexander gave Manne his start in 1938. (Simon, p. 459.) While conceivable that Manne did perform briefly and informally with Alexander in late 1938, this is unlikely, as Manne was just learning the drums and still attending high school at that time. See Brand, pp. 5-7.
The Down Beat poll results are still to be found by searching on the Down Beat Web site but they are much less visible than the more recent "Down Beat Hall of Fame" listing, which does not date back so far. Still harder to find is anything from the polls in the now defunct Metronome and Melody Makers magazines. Kessel, Brown, and Manne also won the Playboy polls for 1959 and 1960 (see Feather, pp. 484-85), a fact no longer easy to discover outside of jazz histories or the liner notes for their albums.