Subwoofer controller uses a single IC TL072, one channel (U1A) is used as a mixer channel Left & Right and also as a buffer for the channel U1B so that the controller is not affected by impedance input that may vary in value, and other channels (U1B) is used as a controller.
In the mixer, the magnitude of the voltage gain of around 1x. Its value is taken from R3 / (R1 || R2), which is about 22k / (47k || 47k) = 0,936x. see circuit diagram
Use this regulated power supply circuit for Subwoofer controller using IC TL072