eventually you do get a square wave (similar to a transistor) but with less overdrive, you get a "soft clip" waveform that is rich in second harmonics. these harmonics are an octave above the fundamental, and can also create a psychoacoustic effect where you hear another sound that is an octave below. bass players LOVE using tube amplifiers for this reason--it really fattens up the sound without making it sound crunchy.