High end stereo preamps? Worth it?

I agree with most of the other, a preamp can make a huge difference. I went from a Rogue Cronus integrated with a nice tube sound to Belles amp and Audio Illusions preamp. the amp provided all the umph and power I was missing but the preamp (tubed) brought that tube sweetness and sound stage that I love. So, yes I believe they do make a difference. SS or tube and cost will be your initial marks. read and get knowledgeable. listening is imp if possible. I bought mine blind but from credible source that also own speakers like mine. Be aware that some preamps have extra features like MM/MC. that could be handy down the line. Happy hunting!

@mikelavigne really said it all in his informative post.   Not much more one could add to his post based on his considerable experience. I have also come to the exact same conclusion.  Well said Mike! 

@fastfreight 

Many have posted how a quality preamp improved their systems. None will be able to tell you the science. To many of us, It just sounds better..

I agree using a preamp (either a unity-gain buffer or an active preamp with gain) sounds better than passive options (and I have owned multiple resistor-based passives, TVC passives, and my current DAC that can directly drive the amplifier using a volume control implemented by adjusting the reference voltage.... out of the signal path).  

Some here have actually shed light on "the science" related to the improvements of active circuitry between the source and amplifier.  The issue is not just a factor of having enough voltage to drive the sensitivity of the amplifier but is also related to driving the interconnect cable, impedance matching, and more. If you search these forums, there are informative posts by knowledgeable folks such as (the late and well-liked) @almarg, and @atmasphere the long-time owner/designer of amplifiers and preamplifiers of the same name.  Nelson Pass has also commented on the subject in his comments about the Pass B1 Buffer Preamp:  
https://www.passdiy.com/project/preamplifiers/b1-buffer-preamp

Wading through the many "passive vs. active" posts here could lead you to some science-based information, but would be time consuming. Below are some comments by @atmasphere on the subject of why active circuitry between the source and amplifier is beneficial:

The reason to use an active line stage is to reduce coloration. The coloration can be caused by the cables and the math that derives therefrom (bandwidth limitations, increased output impedance of the source and thus the impedance driving the amplifier).

Gain may not be needed, but buffering the input and output of the volume control so that the source and amp see a constant impedance is. In this way the setting of the volume control may not also act like a sort of tone control.

The thing is, if you do your design homework, the distortion and bandwidth limits of the active line stage can be much better than those of the source itself and if you do your engineering homework, its possible to insure that the kinds of distortion it does make are of the type to which the ear isn't particularly sensitive.

mitch2

certainly, awesome very advance expertise you share herein. Soooo much brain energy being deployed.  This is complex stuff requiring many advanced degrees.

speaks to a need to improve the marketing efforts of high end hifi devices.  Basically, a large number have no clue what they are buying. They are comforted with expert reviews that seem helpful, and spending more money for incremental perceived value. Choosing a preamp is not easy!!! Marketing materials are horrifyingly tough to make sense of.  As long as boxes are built like tanks, weigh more and look pretty.

@mikelavigne plus 1 gazillion.

There are those who don’t believe that a top notch preamp can make a difference. To them, they haven’t heard the difference to make it worth it. Same goes for all bits are bits and cables are cables. Fine.

When you actually do get to hear the difference in your system, everything will click. I happen to love certain well executed integrateds but in my room, my speakers, my cables, etc a fine pre and a fine amp surpasses.