Well - I can now report back my listening findings between the Nagra Classic amp and Dartzeel.
In short, I now own the Dartzeel CTH-8550 - never ever thought I'd pick up an integrated. Of course, all observations and words here are my observations and ymmv. Source = Lumin X1 and Roon and Qobuz. Speakers = Verity Sorastro 2s. Cabling = Audience SX interconnects and speaker cables to top and bottom, Shunyata Alpha NR power cords and Denali 2s. Room = open to house so not pressurized, main room is ~33+feet long, 15 feet wide, ceiling 8-12ft with big wool floor rug. Speakers on the short wall with big bay window behind, definitely not optimal.
Nagra is all about speed, attack, detail, texture, excitement, fleet of foot and nimble, and yes, musical, too. I'd say puts the listener in more of an observer and appreciator/sympathizer role than the Dart's engaged empathizer role - perhaps overly poetic and silly, but that's what my typing fingers just spit out, so I'll keep it. Terrific image breadth and height. Speakers are nearly invisible. And this thing is Silent. Elegantly made, solid, simple in appearance and operation. The biamp feature is terrific and made the Verity's sing and the bass growl. Never ran out of steam. My nit pick - perhaps too fast, perhaps a bit tipped up, tho that could be the ribbons and the room. My wife said the system never sounded so good. My daughter, a Berklee College music grad liked the set up too.
Dartzeel is all about emotion, grace, depth, empathy, muscular as in a powerlifter, definitely not in a rush, and very very musical. Terrific image breadth and height and depth, too - wow, suddenly that drum kit is a couple feet off the stage and in the back - terrific! Speakers are more invisible because the image is scary good. And it's also silent, tho perhaps not quite as black as the nagra. Amazingly, at low volume, the amp is spooky in it's ability to engage and project an image - how is that possible (and I wasn't even high)? Beautifully made, and not nearly as ostentatious as it looks. A nit pick - could be a bit faster with a bit more attack and texture - perhaps that's a consequence of AB'g with the Nagra.
The Dart is also much more tolerant of poor/older recordings I think in part because it's presentation is biased towards a musical liquid presentation than a detailed fast all-the-information-you-can-handle presentation. Because I want my system to be able to make me weep (happily!) than jump up and down, I'm more in the Dart/tube camp. Yep, it's a bit quirky (pleasure control!) and the manual is entertaining - never actually read a manual from page 1 to the end in a single sitting, but Herve made it fun and laugh out loud funny at times. As a side note, Dartzeel is an anagram for his last name, and his email is an anagram of his first name - quite the character - very responsive to emails. Also, FWIW, Herve said the CTH-8550 mk2 betters the NHB model one separates.
To blather on, for a "true" comparison, I also AB'd the Nagras thru the Dart's preamp section, because the 8550 allows you to defeat the amplifier section and send signal out through RCAs. This tamed the Nagras somewhat, but not by much - which suggests the Dart magic is in the amp.
In addition, at first, I was not enamored with the Dart at all - found it warm, plodding, overly pretty, genteel. Could not understand what all the fuss was about - how could a solid state sound like that? While reading an interview between Herve and Matej at mono and stereo where they were discussing power supplies, I decided to unplug the Dart from Shunyata Denali 2s and go straight into the wall. Viola. The Dart came alive. No idea why, as the Denali was a fabulous improvement for the sources and the Nagras and is not supposed to limit current. Whatever, it works way better - had to start the evaluation all over again.
Also, want to give a shout out thanks to Collin at Gestalt in Nashville for providing me with a laid back informative guiding hand through this process. He contacted me soon after my initial post herein and we chatted about amps, music, wants, desires, and budget. He's a super patient and experienced guy who asks good questions and actually listens to the response, converses easily and unhurried, and offers suggestions without forced opinions - what a great service! And to boot, he was not deterred by the fact that he and I spoke several times months earlier about tube amps and in the end I decided to go a different way. He's a wonderful source of help, insight, and equipment.