The Speaker Breakup slider lets you specify how much distortion is produced by the current speaker model. Increasing the Speaker Breakup setting adds distortion that is a combination of cone breakup and other types of speaker distortion (emulated by the speaker cabinet model). Range is from 1 to 10.
Below certain frequencies, the speaker cone vibrates as one piece. Above those frequencies (typically between 1 kHz and 4 kHz), the cone vibrates in sections. By the time a wave travels from the apex at the voice coil out to the edge of the speaker cone, a new wave has started at the voice coil. The result is comb filtering and other anomalies that contribute to the texture of the overall sound.