How important is a preamp for purely digital sources?

I was using ADI RME2 DAC as preamp (you always have some kind of preamp, passive or active, i suppose. If you're relying purely on digital volume control of DAC, that's the worst scenario since it reduces S/N ratio greatly. Try to avoid that at all cost!). 
Later I switched to tube preamp (Primaluna), and difference was significant. Another upgrade was replacement of ADI DAC, and that brought much more musicality, so go for R2R or mutlibit DACs rather than delta sigma ones (general advice, there are great delta sigma dacs of course!)

The knock against digital volume controls is no longer true.  If properly implemented they will not strip bits or increase noise levels when utilized in appropriate setups.

I agree with Hudo. I have a modwright swl 9.0 tube preamp and the improvement was stellar. Adding the Pontus dac a couple a years ago has made it hard to leave the house sometimes. If you do add a preamp, make it tubes. I have a solid state amp, and the combination of the two really sings.  

Check out the PS Audio videos on Youtube.
Paul McGowan give his take on this.
Here it is:
https://www.youtube.com/watch?v=Dh27E7YKN9s
Bob

You need to tell us what your alternative is - **something** must do switching between sources (unless you are doing all that in the PC/whatever) and control volume. You may, or may not, requires a small amount of gain - its systems dependent (room, speaker efficiency, DAC drive level).
Digital volume control in any PC/MAC/whatever has serious issues that i wont get into here.  And they still exist.  Volume control within a DAC can be very good, or very bad.  In order to perform digital volume control in the DAC; it must operate on a roughly 30 bit or better native signal - some chips do this, others don't. ESS has a nice preso online explaining this and showing the numbers (which are scary). Roon's digital volume control is quite good, but even they admit its not on par with fixed volume. So its a trade-off.
http://esstech.com/files/3014/4095/4308/digital-vs-analog-volume-control.pdf

I'll add one more comment since you used the wording "how much benefit?".  Components dont improve things.  A truly great components simply doesn't degrade the signal..... much. So you are really trading off the evils of the digital volume path vs the evils of an analog volume and gain path.  Conductive plastic vs digital manipulation and loss of resolution.
To give you an idea of the magnitude - were you to use the volume control in a mac or PC, you could easily get below 11 bits resolution, while audiophiles argue whether 16 or 24 are needed.  Holy orders of magnitude Batman!

If your solution does a pretty good to terrific job of digital volume control, and you don't have a really good DAC, I'd probably go DAC by the way - but the devil's int he details!
G