amp current consumption: fixed or variabl

Hi Guys,

Does an amp always draw the same amount of current or does it vary depending on how loud the volume is turned up?   

The reason I ask is that I am running two amps off two marine deepcycle batteries off a single altenator on an outboard engine.   The engine spec says the altentor makes 45A, while one of my amps says it draws 25A, while the other doesn't say, but it it a 300w amp so I am assuming its draws around 20A.  

Thanks,

Tc