Here the circuit diagram of stereo tone control system which also available on the market in circuit kit, you might get the kit at electronic component/part store near your area. The tone control require 12v of supply voltage to work.
The circuit designed based on common tone control circuit, by using two transistors FCS9014 in each channel, so there are actually four transistors in this 12v stereo tone control. Take a note that you need to install the circuit prior to the amplifier device. The output of tone control should be connected towards the input of the audio amplifier. Any small signal NPN transistor such as BC547B, 2N3904, C828, or C945 may be put to use to replace FCS9014.