Synthogy Ivory's key noise, timbre, stereo width and stereo perspective controls can create a sense of distance without artificial reverb. Look for similar parameters in other software.
When you want to move an instrument back in a mix, the obvious approach is to crank up the reverb. But many virtual instruments offer more subtle — and realistic — ways to alter the listener's perspective. For example, Synthogy Ivory (Mac/Win, $169 to $349, synthogy.com) lets you roll off the highs, reduce the mechanical noise, shrink the stereo width and switch to “audience” mic perspective (see photo), all of which enhance the sense of distance. Turning those controls the opposite way brings the instrument forward.
Complete the effect by adding a touch of reverb appropriate for the space: a small room ambience for nearby instruments, say, or a deep hall for distance. As a bonus, using less reverb can make your mix clearer.