If I understand correctly, your computer/dac has too much gain and you can't control the volume. I haven't used a mac in a while but I think you can go into preferences and change the sensitive the volume control. Kind of like setting the mouse up to move faster or slower when you move it. If you don't have adjustments to work with you may have to go with a preamp. I don't know what your budget is but to me, the preamp is very important to get right, and that usually means spending a lot of money because it is very hard to get something that sounds good at a low cost. So, basically, my opinion is if you can't get a really good preamp then don't get one at all. I would go with a passive unit because you don't have to spend a lot of money to get something that sounds very good and you avoid all the problems that a cheap line stage brings with it.
DAC needs a Pre Amp ?
I'm wondering how many DAC / Mac Mini users source into a preamp? I have been using a 45 tube SET amp 1.7 watts per ch and Songbird to listen to my Hi Rez downloads from HD Tracks. Volume control is done with songbird. Today I hooked up another amp about 4 watts per channel and just about blasted myself out of the room. I'm guessing most of you use some sort of preamp or maybe another program other than songbird.