Depends upon the model of car/radio. For some cars you will need a summing device in front of the cleansweep. Some processors have summing built in. Some brands tell you that you can use the car's volume control BUT if the output is not the same over the full volume range, the stereo will sound different at low volume than high (usually less bass at high volume). So, JL is trying to give you a flat signal at X volume on your deck and then when you use the JL knob, the volume can increase/decrease and the EQ curve is the same. However, the clean sweep does not have summing built in, 500 bands of automatic EQ adjustment, time correction, and crossover setting as well as user controls and presets like the Alpine PXE-H660. I have used both. Both work.