How important is a preamp for purely digital sources?

The Pro-Ject Pre Box S2 Digital is a great suggestion. It's excellent as both a DAC and digital preamp. To better this unit with a stand-alone preamp is not exactly cheap. I added an AVA Fet Valve CFR preamp between the Pro-Ject and amp. It improved the overall sound but for many, I think the margin wouldn't justify the cost. 

The Pro-Ject is one the best bargains currently available in digital audio. Dan Wright of Modwright Instruments was very impressed by it:

 https://www.audiocircle.com/index.php?topic=158442.0

My experience is similar to Paul McGowan’s of PS Audio. I used to follow his mantra of “there’s no preamp like no preamp.” Paul’s opinion famously changed when he listened to an Aesthetix Calypso in a friend’s system. 

My experience has been similar with my BAT 51se. I get absolutely beautiful results when going direct from my PS Audio DAC to my Pass Labs XA30.8, but the 51se always adds something. 

Right now, I keep the preamp at unity gain, and do all of the volume changing on the DAC. I’ve resolved to sell my preamp several times in the past year, and put the money into other equipment. Every time I pull the preamp out, I miss the dynamics, the 3D imaging, and the richness that the BAT 51se adds. It doesn’t stay out for more than a couple of days.

Functionally, my DAC is the preamp. But the sound is improved through the 51se to such a degree that I just haven’t been able to bring myself to get rid of it, as strange as that may sound.

I've asked the same question and came to this conclusion - 
1) Is it necessary?  No.
2) Will it help improve SQ?  Very likely, assuming you pick the right one, which will involve time and money.  But it might not.
3) Most importantly, is is the best next improvement for my system?  Depends on what you're hearing and want you want to improve.  

Lots of good input based on others experiences, which can be helpful, but I think OP needs to share more specifically what their goals are to get the most valuable feedback.  

When I asked "what would a preamp do for me?" my conclusion based on forum feedback and my own research was to work on room treatments, isolation, power, and upgrade my source.

When I asked if upgrading my DAC was a good next step, my conclusion based on forum feedback and my own research was to work on power, consider different speakers, or to rebuild the XOs.  

This can be a strange and enlightening hobby, it's not all about buying a new black box.

I find a system sounds better with a
Preamp if you are going digital 
look into R2R Ladder Dac 
with a Jays Transport 

Well, oddly [to me, anyway], no one mentioned this, so I will ~ as observed, w/o a preamp, things do often sound a bit harsh...so I use one w/an added feature: EQ. Several spoke of the preamp "adding something"...why not be more selective about what ~ and where, in the frequency range ~ you are adding? Or subtracting, when necessary...My C48 has 5 band EQ, which allows me to adjust for my room acoustics and system synergy...the only way I would ever relinquish it would be for a c52, which has 8 bands of EQ, rather than 5.  Purists, please don’t bother ~I don’t have a perfect anechoic chamber for a listening room, and if you want to be stuck w/whatever sound comes out, and that sounds good to your ears, great...I’ve done some room treatments, which help out, but it’s basically a crap-shoot and it is SO much easier, faster, and less expensive to make a slight adjustment to a few dials...and this way [unlike w/digital room correction], I can tweak to my room/speaker response/ears.  And, I’m using a Berkeley DAC, so I really don’t think I need EQ because my DAC is so poor...As a final observation, I find it interesting [and refreshing] that some of the writers of TAS, Stereo Review, and others are now swinging around the other way, and are openly in favor of a judicious use of EQ to enjoy their music, rather than viewing it as some anathema of evil to be avoided at all costs...try it, you might like it ;-)