How important is a preamp for purely digital sources?

I have an MSB Analog DAC with a "stepped attenuation" volume control and power base, coupled with a PrimaLuna DiaLogue Premier HP.  I think that the combo sounds great and replaced my very excellent and modified Plinius pre.  Seems to me that logically and unscientifically, adding a preamp to mix just adds more "stuff" to the signal path.

I would like to suggest you examine using an Exogal Comet it’s both the a DAC and PreAmp.  It’s a SS DAC that many are using in combination with their existing Tube or SS Amps but it really shines when you combine it with Exogal’s Ion amp.  For more specific details check out Doug Schroeder’s Dagogo 2 or 3 reviews.  I have both Comet & Ion and knew in less than a minute of listening that it punches way above its price point and is a game changer sonically especially at its price point!

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.