How important is a preamp for purely digital sources?

All components have a value.  It comes down to how they work together to improve the sound.  Asking a general question like this will give you multiple answers from different member experiences.  Any old preamp may not be a benefit.  A DHT preamp may add wonders to the sonic improvement but how much and what you are trying to spend also can be a factor.

I have heard systems both ways and I prefer using a good preamp.  The sonic improvement to me was substantial in adding to the depth of the soundstage, clarity, musicality, micro to macro dynamics and back again, layering of instruments and vocals, and the piano and horns sounded "real" adding to the overall pleasant experience.  The direct way was not bad to say but the improvement in these and other areas was what I preferred.

Happy Listening.