The audiophile world is a bit divided on this one. A significant number of people believe that no digital volume control will ever be as good as an analog volume control.
Personally, I think that within reasonable parameters, a very well executed digital volume control (like that found on either Pure Music or Audirvana Plus) can be as good as a very good analog volume control. I think you need to have a very good preamp indeed to do better than, say, Audirvana Plus. That's not to say that I believe Audirvana Plus can dispense with the need for an active linestage -- if there's benefit to an active linestage, a well executed digital volume control won't obviate that.
With 16 bit music (let's face it, what most of us have) and a 24 bit DAC, you've got at 8 bits of information you can use for volume control before loss of resolution kicks in. With a 32 bit DAC, you've got 16 bits you can use for volume control. There's a bit more to it than that, but you can read more here:
http://www.izotope.com/tech/mbit/This is the dithering that Audirvana Plus and Amarra use.
I agree that the volume control on iTunes isn't very good. Amarra reputedly has a good volume control, but unless you've already purchased Amarra, I cannot in good conscience recommend Amarra to anyone. It's too buggy and too flakey. Audirvana Plus sounds every bit as good, is more stable, and costs less money.
If you're thinking of using a passive preamp, most passive preamps don't have remote volume control. You said that your active speakers have volume control -- isn't that likely the same as a passive preamp? Wondering what's wrong with the analog volume control you already have on your active speakers ...
As for active line stages, I quite like the Modwright LS-100. But that's a $3400 preamp. It will add body, 6SN7 overtones and texture, and a drive that might be missing if your system would otherwise benefit from an active linestage.
