Switcher needed

I have a small tube amp a friend made for me. I guess it's integrated in that it has one input, a power button and a volume knob. I'd like to get some kind of a switcher to select between different inputs. Need a min of 3 inputs, maybe four. Anyone have an ideas?

If I went with a pre-amp, how would that work? Would I just leave the volume of the tube amp at max volume at all times?