The following diagram is the circuit diagram of low cost 12V stereo tone control which also available in kit, you may find the kit at electronic part store around your place.
The circuit build based on ordinary tone control circuit, using two low cost transistors FCS9014 in each channel, so there are will be four transistors in this 12v stereo tone control. Take a note that you must connect the circuit before the amplifier module. The output of tone control must be connected to the input of the amplifier. Any small signal NPN transistor such as BC547B, 2N3904, C828, or C945 can be used to replace FCS9014.
There are will be four potensiometers used for this tone control circuit: volume level, bass level, treble level and balance adjustment. For better result, use better components with small tolerance value for example metalfilm resistors, MKM capacitor etc.
This circuit powered with 12V DC power supply, you may use regulated power supply for better performance. Just use regulator IC of LM7812 to get regulated voltage. 