if you know anything about muscial theory, theres this thing called the Pyramid of sound. Basically it says that you need more low than mid, and more mid than high. so just mess around w/ that until you get somthing you like.
And as many above me have said that you can "ruin" your bass amp by using an eletric on it, that is false. There is almost no difference between a bass amp and a guitar amp, other than one has distortion and one doesnt.
I just plug a Digitech sound board into my amp and use it with my eletric and my bass and its fine.
hope that helps