High end stereo preamps? Worth it?

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).

Worth is a subjective question.  Relative worth may be easier to deal with. I always ask the question "if i have $1000 to spend, where do i get the biggest benefit".  That depends, basically, on where the biggest problems are. And its exactly why i'm leery of most expensive tweeks, including cables and similar doo-dads.

Speakers traditionally have the greatest compromises. Unfortunately you need not only money but tolerance for size to move up with speakers.
IMO sources (turntables with carts) and DACs are also ripe for improvement. And both are fairly complex so it make sense.  As i work on DAC designs, the number of small to medium pitfalls is large. power. grounding. Isolation. Filtering. Blah, blah.  never-ending.
The best amps and pre-amps ought to be nearing perfection.  nearing, not there.  Sadly, many are not.  But the declining marginal returns are in full force over $2k for pre-amps IMO. Amps are more complex to answer since some speakers, in some rooms demand lots of power - and power you buy by the pound to a degree (transformers, output devices, heat sinks, and a chassis to hold all that heavy stuff!)
Right now i have a prototype integrated making the rounds. Its ~ 40w/ch/8 ohms and is targeted to sell for $2000-2500 "depending". It has qualities where it is superior to and others where it is slightly inferior to, $7-11k worth of separates. 

Now, nothing i design will "wow" anybody with lots of seductive distortion.  That's not my thing.  I want it to disappear, erring toward "never be obnoxious". So if what you want is a ton of euphonic harmonics, go for it, but that's a 'whole different kettle of fish.
But the best answer is - listen and trust your ears. As a great sommelier once told me when i asked the best way to really learn about wine: "pop a lot of corks".
Happy drinking.
G