Volume problem with Passive Preamp

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.