The schematic for the Box of Rock is shown below. The BS170 is an n-channel MOSFET (and works differently than a JFET like the J201). A MOSFET like the BS170 used in the B.O.R. is an enhancement mode device, i.e. current doesn't start flowing until the gate voltage - the source voltage (Vgs) is greater than a threshold voltage (Vth) which is device dependent. Basically as you drop the source resistance, the source voltage drops and Vgs > Vth and the device turns on. As the source resistance drops, the gain increases. Another way to achieve this in a frequency-dependent way is to bypass the source resistor with a capacitor. Note that you have to bias the gate and add the diode for protection.
Also note that you could also lower the 330R resistor and increase the gain of the third stage. If you really want to play around with this to your heart's content, get a breadboard and set up the circuit so you can plug in different source resistances. Have fun!
More here:
http://www.electronics-tutorials.ws/amplifier/mosfet-amplifier.html