JL Audio HD-RLC Amplifier Remote Level Control and Cable The HD-RLC remotely controls the Bass Level of the following JL Audio amplifiers:HD-Series: All modelsXD-Series: All modelsMHD-Series: All modelsM-Series: M600/1, M200/2, M500/3, M400/4, M700/5, M600/6 When the HD-RLC is connected to the above amplifiers, it acts as an attenuator on the input gain of the amplifier and is used to adjust the overall level of the amplifier from the front of the vehicle. The HD-RLC kit includes the rotary control with a black ABS knob, mounting hardware and one 18 ft. (5.5 m) cable.