1. 20 dB reduction means it turns down the volume by 20 decibels. That's a big reduction in volume. You use that reduction to help balance the strength of the input signal so that you can get a good balance of volumes across the board. For example, different microphones put out different amounts of power, depending on what kind of microphone it is. You use the reductions (there is prolly a 30 dB reduction in there as well) to damp down the louder mics so that the quieter kinds have a chance to compete.


  2. More later, gotta go to school now...