High end stereo preamps? Worth it?

Yes, they are, in answer to posted original question.  But the dac direct is a nice fallback if cost prohibits getting a good preamp.

@fastfreight 

In each setting, the overall sound and especially soundstage was improved by inserting a preamp.

My experience exactly after trying multiple types of passive volume controls and DAC direct to amp approaches.  I am using a unity-gain buffer, so no gain like a traditional preamp but active buffering of the signal, which seems to preserve the tone and dynamics that I find missing with passives and DAC direct approaches.  To answer the OP, "high end" preamps presumably handle the signal better than lower cost approaches (i.e., better design, better power supply, less shortcuts, better parts) but more dollars don't always equal good sound so to your question of "High end stereo preamps? Worth it?" my answer is, having a preamp or buffer is better than having none, but to the value - it depends on the preamp. 

Wise words mitch2.  Of course, if one has multiple inputs (beyond your DAC), the decision is easier.  I actually only have two digital inputs; ROON and my TV, both into my DAC.  Yet, to me, the preamp is indeed worth it.  

My comparison was they are not. But not quite the way the question is worded, which is asking if its worth separating the preamp from the DAC, etc. That part is definitely worth it. What I mean is not worth it is preamps in general. 

Not because you can't get better sound quality that way. You can. If that's all there is to it then go for it. But, "worth it"? Compared to what? That's where they come up short. 

Compared against each other everything seems fine. Compared against a good integrated however suddenly you realize it takes a pretty darn expensive interconnect between them or it just won't cut it no matter how good the preamp. Also both the amp and the preamp need at least as good a power cord as the integrated has for the comparison to work out at all. If you use a shelf, Cones, or whatever, add those in too. It all adds up to where you've spent a pile of money all just trying to get the preamp/amp to keep up with the integrated. 

But in terms of worth it to separate the preamp from the DAC and phono stage? Yes, totally. Just like in a high end integrated.