Mike, the level controls(not "gain" controls; audio amplifiers have fixed gain)on some amplifiers, especially Crown and other pro amps, work the same way as other volume controls: a variable resistor allows varying amounts of the incoming voltage through to be subject to the fixed gain(typically increasing the voltage by a gain on the order of 30dB, about 25-30 times).
As to whether there are units that connect between e.g., a pre-amp and amp to reduce the voltage on one or more channels, see
this fancy(about $135)volume control with a vaguely familiar name.
