The most important is the gain.
The purpose of the gain is to match the signal volts RMS coming from the source (CD player, etc.) to the input of the amp to prevent clipping (damage by over powering).
Here is a guide that will help you set the gain correctly http://www.box.net/shared/uchv4dbk88
You'll need a multi-meter (AC voltmeter, $15 from Radio$hack), Microsoft Excel and a way to burn an audio CD from an MP3.
If you don't have Excel, use this link http://www.box.net/shared/nkkw1dhk4g
See my site for more info http://spkrbox1.spaces.live.com