High end stereo preamps? 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.

I think there are...the question is related to return on investment.

I have a Moon DAC by Simaudio and they make impressive DACs that have preamp capability including MC/MM phone inputs as well.  My preamp is a McIntosh and they have quality DAC and Phono sections.

You can get a quality pre-amp with DAC and phono stages and you can even explore a quality integrated amp.  There's a trade off in costs and theirs advantages in simplicity including interconnectors and space requirements. 

A really good preamp is one of the most important parts of a system. I can still remember the first time I inserted a really good preamp, Air Tight ATC-1 into my system and being stunned at the transformation.

As others have suggested a good preamp doesn't have to be expensive, but they usually aren't cheap either. Buying used is always a good way to stretch your dollar.

There are always compromises to all in one designs.  For example my DAC has a 30 lb power supply, preamp separated power supply so all in ones don't compete.  That is not to say that all in ones cannot sound good.  The Vinnie Rossi all in one is a very good sounding unit but it is also a DHT design which is what I prefer.

Happy Listening.