Volume problem with Passive Preamp

If your volume control is a stepped-attenuator you can change values to make the steps closer in your desired listening range. This would take some work. Open your preamp and look what's inside. You could also change the attenuator to a model with more steps. Probably an easier and better option if room permits. By the way you will need to know if your attenuator is 50k, 100k or 250k

You can add a series resistor as Elizabeth mentioned and that will help with the first step. But unfortunately it will not correct your problem with steps being too great as mentioned.

You can go to this website and look at voltage dividers and figure out what attenuation you want on the input. Two resistors one is series and one to ground. But this will not make your steps closer together.
http://www.facstaff.bucknell.edu/mastascu/elessonshtml/Resist/Resist3.html