Below is a cheap high fidelity (hi-fi) tone control circuit. This circuit has one channel (mono) control and we need to build another similar circuit to make it stereo. For better sound quality output, use good component (such as metalfilm resistors, MKM capacitors, tantalum capacitors, good cable quality) and stabilized power supply. The power supply, cabling and quality of component will affect the quality of the output.
Transistors is used here since they’re cheap but give very low noise performance. The center-tapped linear potentiometer is used to get better logarithmic approximation with cheap linear potentiometer. R16 and D1 (12V Zener diode) make it possible to accept wide range of power supply voltage (12-24 V). If you use an unregulated 12V then you can lower the zener to 10V to enable ripple suppression by the zener diode.