Hmmm, let me describe it this way... Unlike a purely passive setup, the Io runs the line level input through its final gain stage just before the volume control. This gives the buffering of an active line stage, so you don't experience the potential dynamic limitation that sometimes can occur with a purely passive volume control when impedences are not ideally matched. If you like the Io and find it's sound well detailed and transparent (as I do), that's what you get using it as a linestage by adding the volume control and high level input.
The volume controls are the best available. Yes, there are two of them, but you quickly get used to just grabbing one in each hand and turning them in tandem. It will be hard to find any $2500 line stage that has less coloration than the Io with volume controls. In fact, what you're paying for in the Io will be those volume controls - they alone cost as much as some other tube or ss line stages.
Hope that helps some.
