Typically...even if the head unit says "4V output" it's nowhere near that. Not because it's a "crappy" deck; but because music is DYNAMIC. Set gains and leave it be.
RCA output voltage, nouseforaname
EDIT: Go back and try what I said already...hook up an ipod or something to it and see if the sound is similar or dissimilar. Then you at least know if the problem is upstream or downstream.