On the SD1 I'm using it differently than most as I hate the way the amp is pushed setting the volume on max and gain at minimum. I set the volume where it doesn't alter the volume and I set the gain at approximately 8:00-08:30. To me that sounds the best, tightening up the sound without flattening it too much.
To many people up the settings which squish the sound, making it over the top. The same goes for the gain settings on the amp. Less is more coming to the overdriven sound. Up the gain will make it sound thinner and less defined. This is really notable double- or quadruple tracking. Using a lower gain setting will make the tracked guitars sound HUGE and razor-sharp defined.