Value of a preamp?

Arguable, all preamps "color" the sound.

Instead, I prefer "season" to "color," since that’s all any audio equipment is doing -- presenting artifacts of sound for delectation. Nothing is untouched. It’s all artifactual.

I have tried using my DAC as a preamp. My first realization was that the max volume was much lower. 

Secondly; my preamp "added" something. In my rig's case it added brightness- which I do not prefer in the room I listen in. Perhaps a more absorbent room would change my opinion. 

Phono

To my ears the preamp is the make-or-break component in my system.  A good match expands the soundstage and makes instruments appear more solidly defined in space, with tone colors and timbre faithfully reproduced….a bad one sounds flatter and more grainy. I discovered long ago that the preamp sections of well regarded Japanese brands were the weak link holding back the amp sections of Sansui, Pioneer, and Sony integrateds.  That’s why I’ve never owned one, always preferring separates, even economical ones like Dynaco, Crown, and DB Systems.  Then I discovered Conrad-Johnson.

with exceptions, system sounds better with it than without it.