High end stereo preamps? Worth it?

The music had structure and substance I never heard.

You probably heard tube microphonics/euphonics.
If you put that same source "direct" into a pair of very expensive headphones you will get the sound more similar to the passive pre than the tube. A tube cannot make music it can only amplify and in this case "color it"

Cheers George

@djones51,

unfortunately a digital volume control progressively leads to a loss of resolution as the available bits get truncated. As long as you stay above say 80% of max volume, your statement applies; below, not so much

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.