very sorry for the dumb question but im thinking about getting one of these and been reading alot about them. but i do not fully understand the gain control thing. i have a pair of modded Grado 325is and will be using a ipod 160gb classic. is this basically a listen and change (opinion) thing or certain headphones will be set at one thing and grados 325 at one thing ect.......... please help me understand in kindergarten terms 
thanks
thanks