Adding Bass and Treble control to existing schematic
Here are two circuits you could use:

http://www.diyaudio.com/forums/parts/144872-help-understanding-tone-circuit.html

Whichever you choose, you will need two of them, one for left, and one for right. Connect the output of the tone control circuit above to the input of your amplifier circuit. Use dual pototentiometers so you can have one treble control, and one bass control, each of which will affect both left and right channels simultaneously.

Here are some other bass / treble circuits:


http://www.electronicecircuits.com/electronic-circuits/bass-treble-tone-control-circuit
http://www.ampbooks.com/mobile/amp-technology/james-tonestack-synthesis/