High end stereo preamps? Worth it?

Yes, but as always it all depends on what sound characteristics you’re looking for.  There’s also VAC, VTL, LTA, Shindo, Pass Labs, Rogue, Backert, Allnic, Ayre, Atmasphere, Conrad Johnson, Bryston, etc. that are also excellent.  Again, just depends on your personal tastes and system synergy.  

I tried direct with DAC to amps, using several DAC's touted as having good preamp volume control: PS Audio DS with bridge, Auralic G2 and now my Mola Mola Tambaqui.  In each setting, the overall sound and especially soundstage was improved by inserting a preamp.  First the PS Audio BHK Pre, then moving on to the Audionet Pre 1 G3, and finally my nirvana, the Audionet Pre G2.  My measurement and science buddy points out that a preamp should not be able to add anything beneficial, just take things away.  My ears tell me differently!  I wonder of many people that recommend direct to amp have heard the difference a good preamp can make.  And +1 to soix3 on synergy. While the Audionet Pre made a difference, mating it with the Audionet amps was amazing. So while looking for that quality preamp, and many good mentioned here, keep your eye on the same company's amps...as in one day 'I will add those also'!

Only if you have an income 10X the price of that expensive preamp! It's really only jewelry!

I have limited owning experience but did a lot of research on preamps. The Quicksilver Line Stage, non-remote, version, is very simple, point to point wired, very good customer service, and has a very good reputation with customers. The company does not send units out to reviewers and is made in California. It's $1500 new but can be had for close to $900, used. That is, it seems to me, an extraordinary value in audio. 

+2, soix.

Once you hear Luxman, Accuphase, LTA, CJ, VTL and Shindo, you will know exactly what you were missing in your music.

BTW, Accuphase Integrated have an option to add phonostage and DAC modules.

In a highly resolve system, everything matters not just a high quality preamp.