Some power amps have an input sensitivity switch or control knob. In your case it's a knob which makes it a bit harder to setup. You want the gain of each channel to be the same and set so that the full volume of your preamp produces the full volume of what the amp is capable of. The setup method I'd use is probably not the one you would as it involves a signal generator and a scope.
The method most use is to play a 0db test tone and adjust the amps sensitivity looking for a target voltage at the amps full output. You can google this. But the quick take is disconnect your speakers, set your preamp to about 80% volume, play a 1kHz tone (you can get files off the web), and measure the AC output voltage of your amp (remember to set the meter to AC). In your case if you have a speaker with a nominal 8 ohm impedance ... then the target voltage would be ~28 volts ... sqrt( 100W x 8 ohms )
Have fun
