Updated September 2, 2010
This is a FET-based bass boost and overdrive. The treble control allows you to add in high end to the bass boost, going from full bass boost to a slightly scooped sound. This is nice for having a bass boost but enough treble so that it doesn't sound muddy. The clipping switch sounds very good, especially with the treble control at around half. If you only want the bass boost, the diodes and switch can be removed.
I've made a few changes from the first version. The FET has been changed from a MPF102 to a J201, which, in this configuration, gives more gain. Rather than just draining off the treble, there is adjustable treble boost, which gives more clarity except when the treble control is at zero. Finally, the gain control has been removed, as there isn't much of a range of clipping.