I'm a composer and frequently write fugues on scratch paper to practice my counterpoint and ability to compose away from the piano.  The problem is not a lack of understanding on my part.  I can hum the tune of the Bach's Art of Fugue.  Likewise, you could probably hum the tune of "A British Tar" from Pinafore despite it being polyphonous. I'm saying that it is not necessary to hum every voice concurrently to claim to be able to hum something, especially since a fugue only has (usually) one or two melodies despite it having multiple *voices*.