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

I had the same issue with a passive unit I have ... I had it modified with a step DOWN transformer and it worked fantastic! definitely not an inexpensive way to go

Buy an amp which you could adjust the L/R sensitivity. (Such as certain Rotel amps.) I had the same issue w/ a Nakamichi pre-amp as well.

Buy a 10 db attenuater, this is a simple RCA/RCA or XLR/XLR device the reduces the volume by 10 db. If you are running a RCA cable, let me know via e-mail as I have a pair sitting around for a time in the past when I had a similar problem.

Just so you know, this doesn't change the steps with each volume control turn (or push of the RC). But if you want to try this approach, email me and I'll send you my pair. If it works to your satisfaction you can keep them (buy them). I think the going price for a pair is $50 used (if they are advertised currently for less, you can pay the lower advertised price). If you don't like the results, just send them back to me.

What's the Vout of the source? What's the gain of the amp? Are either adjustable?

Parts Express sells in line attenuators (HARRISON) for about $35.00 a pair. They come in 3db,6db and 12db of attenuatuion.

Get a resitor DECADE box and experiment with the value until you get the level and range you want.
Than, get good resistors of the correct value.

It is a TVC correct?

Many of the aforementioned methods would work, but may interfere with the impedance characteristics of the device. That would in turn negate many of the TVC benefits. There may be a way to rewire or adjust it. I would take note of exactly what steps are too big, and call them.