ok try this
just to be sure we mean the same thing when we say turn down the gain
unplug all rcas going into amp, turn the gains all the way down (anti-clockwise)
so you have power, ground, and the remote cable going into amp ONLY.
at this stage head unit volume makes no difference, since there is no audio signal gooing into amp.
if you hear hiss with the gains on all 6 channels set to the minimum, i'd guess your amp is faulty, only way to test it is to set it up on test bench away from the car, so you can tell if the noise is being picked up inside the car, or if its being generated inside the amp itself.
just out of curiosity, try setting all channels to low pass, and tell me if you still hear the hiss.