MuBu is a Mu Follower voltage gain booster using 2n7000 MOSFETs. The value of the capacitor coupling the bottom and top MOSFETs has an effect on the circuit's frequency response. If it's too low, low frequencies will be attenuated. If it's too high, it will take a while for the top MOSFET to bias properly. The biasing resistors will also have an effect. If you reduce the value of the biasing resistors, you will reduce gain and at the same time have a flatter frequency response. You may find similiar results by changing the value of the bottom MOSFET's biasing resistors, however, reducing these values will lower the input impedance. There's nothing perfect about the values I've chosen, and one may find better values. Also, one may benefit by using a smaller value for the volume pot. I've got a bunch of Radio Shack 1Meg pots, so that's what I used.