I probably should have specified what kind.

Mostly I do classical because I am just learning. By coincidence of their form, the jazz pieces I know how to play don't sound as interesting with it.

The specific problem is that it's somehow not calibrated to handle polyphony correctly, so when you have just one note, it comes out fine, but when you have multiple they sort of crowd each other and it creates distortion, with more notes being more distorted. Because of that, when a song has, to use the example that has the best effect so far, a monophonic melody but then the accompanying notes vary in number, the melody gets more and less distorted and it adds a new dimension to the song. With the songs I play, which tend to be mildly positive, it tends to sound very somber in an almost mocking way, partly because the soundfont is that of an organ to start with.