Value of a preamp?

It's a specialist.  Volume control and source selection without degrading signal.

I added a preamp to my integrated amp and that opened my eyes to the impact of a preamp.

I'm an audio troglodyte. I like tone controls and most preamps have them, at least the ones I own! 

A good tubed preamp will greatly enhance your sound period.
 

kennyc: I disagree, as there are satisfying SS preamps also such as Dartzeel, Audionet, Griffon Commander…  it depends on the execution and of course user preference.

There is nothing for kennyc to disagree with here. There is no implication that SS preamps cannot also be good. 

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.