High end stereo preamps? Worth it?

Analog  volume control is generally better. The truncated bits is true when dealing with 16 bit DACs not so much with 32 bits. 

@georgehifi, could this be harmonics ... ??? ... "color it"

I used a Marantz AV-8802A pre/processor as my preamp and home theater system processor.  I knew it was a very good theater processor but I also knew there was better hi-fidelity I should have for my musical, 2-channel play time.  I had already changed my amplifier from Adcom to Balanced Audio Technology (that alone was a revelation in sonic superiority to what I had).  I researched further and landed on the Balanced Audio Technology VK-33SE preamp.  It made sense as those units were designed/engineered to work properly with one another.  WOW!  was that an upgrade in sonic superiority.  The soundstage and imaging became so lifelike and dimensional.  Yes, a top-end preamp does make a very significant difference for high fidelity music reproduction.  Do your research to find a good match in preamp & amp and you will be rewarded very well in musical enjoyment.  Note; the BAT preamp does have the capability to change a source input to a fixed "unity gain", basically a pass-through.  This allows you to use an external pre/processor for your theater enjoyment while still using the main amp to your main speakers and not having to change any cable connections.  Very simple, convenient, and very nice.
Best of luck to you in your quest.

@georgehifi, could this be harmonics ... ??? ... "color it"

Yes maybe more like the "harmonic distortions" from the tubes themselves, which some like😏

Like I said before, they can’t make what’s coming out of the source better (add additional music content that’s not there), they can just color it with distortions and or microphonics.

Sources today have enough output to drive any poweramps to full power and more, this is why Nelson Pass said this.

“We’ve got lots of gain in our electronics. More gain than some of us need or want. At least 10 db more.

Think of it this way: If you are running your volume control down around 9 o’clock, you are actually throwing away signal level so that a subsequent gain stage can make it back up.

Routinely DIYers opt to make themselves a “passive preamp” - just an input selector and a volume control.

What could be better? Hardly any noise or distortion added by these simple passive parts. No feedback, no worrying about what type of capacitors – just musical perfection.

And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”, is a commonly heard refrain (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp.”

Cheers George

The comments about additional pieces can only degrade and digital volume controls are all that is needed are just for those systems where they tried it and liked it. I have done everything from using the tape out of a Superphon basic back in the day to bypass the analog volume controls to switching the digital vs analog of the Mytek in and out to using the MSB with its resistor ladder array. It all depends on the surrounding equipment and how they interact. Currently the MSB dac goes into Atmasphere MP-3 with NOS tubes and into either RM200 or Marsh A400s or hotrodded Acoustat TNT200, all balanced connections. Even then, the power still gums up the works some days. Oh well, when it is good it is really sweet! So in the end, you just have to try it to see if it helps!