Then, being careful not to turn the knob, position the lid where it belongs and press hard enough to make it lie flush with the back of the amp - overcoming the pressure of the spring clips at the other end of the battery.
When you've got the lid sitting flush, turn the knob to extend the tabs and smile!
By the way, the jumper inside is not for gain. I recommend you put it back where you found it. That jumper adjusts between high and low current draw. If you want the battery to last longer at the expense of sound quality, select the Low setting. Otherwise, leave it on High.
Gain is controlled by the externally accessed switch that's under the volume knob. Go to High gain only if Low gain isn't loud enough with the volume turned all the way up. Excessively high gain can lead to audible hiss with more efficient headphones or IEMs. The blackest blacks are had with the lowest acceptable gain setting - true for nearly all amps.
Edited by zilch0md - 8/30/13 at 8:44pm