High end stereo preamps? Worth it?

About two years ago I got a very good deal on a used Soulution 720 which includes the phono stage, and is a world-class pre-amp.  I can honestly say that this was one best system upgrades I have ever done, and that is factoring in the cost.  When I consider the value I got for selling my separate preamp and phonostage on Audiogon, this was a 4-figure upgrade and not an unreasonable expense.  The improvement on all of my sources was tremendous, but the phono upgrade ended up far better than I had expected.  To the OP's question, I think that the right high end preamp can make a VERY large difference in system performance.  However, it depends entirely on system matching and what specific role the preamp is expected to play.  That is my experience at least...

There is an Audio Research LS2B MKII with remote on eBay that would probably smoke anything new under $6K.  I've had mine 28 years and finally gave up trying to shoot out something better a few years ago.  Right tube and cables and it's really breath taking......   amazing sound stage and imaging.

I recently picked up a Tag McLaren preamp and I am tickled with the sonics and build quality.

Shindo Monbrison changed my life.  I could barely afford it even used but have never looked back.  It added richness and texture that I didn’t know I was missing.  

I'm not sure that it is accurate to say that a good preamp makes the sound better. It is probably more accurate to say that a really good preamp allows what was already there in the signal to get through better than a cheap one will. It is closer to the ideal "straight wire with gain" 

Try borrowing a really good used preamp from a local dealer on the basis that you are interested in buying it if it improves your listening experience, and  see what you find out.

And it doesn't (necessarily) have much to do with whether the amplification is solid state or tube (I run one system with each and find them both to be very good).