Adding Bass Cut Switch on Pedal
You'll want to add a high-pass filter similar to the one that's in most guitars.

You'll see in this sample (thanks Google!) that they have a variable resistor VR1 that adjusts the amount of bass.  If this was a switch, when closed, you would have a straight feed and when open, the output would be have the bass filtered.  By changing values of C1, you would change the frequencies that are passed.

For your pedal, you could recreate this design, with a switches for VR1 (bass) and VR3 (treble).  You won't really need VR2 and C2 (volume).