If you don't have the physical booklet, here are the primary controls found on most EEQ models (like the DEH-series):
The Loudness function compensates for the human ear's inability to hear bass at low volumes.
You turn the volume knob, but the sound is only coming from the front doors. You frantically press buttons, but the station changes instead. You need the manual.
Press and hold the (Source) button until the unit turns off.