You should use the least amount of gain that makes your cans play well / loud and not hurt your ears or damage you hearing....
The input source and headphone sensitivityhave to be considered when setting up the gain.
Most folks have no idea on how this really works...for most stuff the standard gain settings this amp is built with is fine.
You want to be able to be able to have your cans reach there full potential.
There are other places on the web you can read about gain, and volume and how they are related.
Edited by adydula - 9/11/12 at 4:59pm