Who needs a Preamp??

 generally a shorter signal path is better as long as you have the essentials for proper sound reproduction and a good preamp is one of them .it's critical for depth imaging and soundstage and helps you adjust the sound to your taste. the wrong preamp will degrade the sound and take you away from the sound you like.
a preamp can soften a digital source and make it listenable
tube preamps are the best for that . adding a gain stage with volume control allows you to keep all the volumes on other components like dacs at their sweet spot so it improves sound quality.on some recordings i keep the dacs volume high and the preamp low some sound better the other way around it's more flexible vs having no choice about the dacs volume.you just have to pick a high quality preamp that you like what it adds to your system.if you like your dac's output stage that much try a preamp from the same brand it will give you more of what you like.  

Preamp has always been better in multiple systems for me. 

If your source has a high quality volume control (as with a few DACs that change volume by adjusting the reference voltage), you can achieve the impedance and power supply benefits mentioned earlier using a high quality, unity gain, buffer, which I believe is similar to what @grannyring‘s well-regarded The Truth preamp is doing, although The Truth provides the volume control too using photo cells.  Pass is correct about having sufficient gain in our sources but he also recognized the sonic benefits of an active stage, which is why he designed his B1 buffer.  SMc’s VRE-1 is another excellent unity gain line stage (with volume control).  Tortuga Audio’s tube preamp buffer is another buffer option (no volume control) at a lower price.

I've tried powering a Jolida JD100 tube CD and a Meridian 506 CD, both of which are respectable CD players, directly to my solid state amp that had volume controls for each channel. They both sounded flat to me. Uninvolving. After placing my Conrad Johnson PV5 preamp between the CD player and solid state amp, it made a world of difference IMHO, not just to my ears, but for every single person who listened to my system. It was just MUCH more musical and involving. I guess it depends on the equipment and maybe the music you listen to, since I've hooked both CD players up to a Conrad Johnson CAV-50 integrated and though that too sounds much better than the solid state without the CJ preamp, it doesn't quite have the same sound I prefer compared to the solid state mated to the CJ PV5. Definitely more slam and low end power with the latter. I've never had a truly high end solid state preamp, so I can not comment how that would influence the sound from the solid state amp connected to the CD player.    

a preamp can soften a digital source and make it listenable

This then is the fault of the source, and not the fault of going direct.

Better to fix the problem (get better source) and not just put a band-aid fix over it, by softening the problem with adding an expensive preamp.

Ivor Tiefenbrun, inventor of the Linn Sondek LP12, had a very good saying.
"It all starts with the source, get that right and your part the way there.
Don't get source right and it'll be a never ending struggle"

Cheers George