No, it's not just epic THD, it's the choice of even and odd harmonics, methinks...probably even that is an oversimplification (believe me, I've played with a variety of distortions -- one of the good things about the Alesis Quadraverb GT is that it has several different kinds of analog distortion, which all sound different...and no, I have yet to hear a digital plugin that sounds anything as good as a good analog distortion).