go to stewmac and look up how to build tone control pots. It'll give you the run down for how they work, which is essentially a low or high pass filter.
Try figuring out how to use a high pass filter, and then from there you can get a grasp of " Nth order filters", which is basically daisy chaining filters, to get what you need.
If you can breadboard it, try a high pass filter at various areas. Before the transistors. After. right between the decoupling cap and the output, etc.
