I was curious if that tape amp used a transconductance amp or whether the reference was to servo-based DC coupling. I originally rejected the idea but a servo can do a lot to simplify the audio path, at the expense of DC control complexity. I got that current source circuit from Camille before Valve existed. I loved that guy. He really schooled me on grounding and noise, even beefed up a Tek 7000 scope so I could get rid of gremlins in some radio gear I was working on. That's how I learned that almost all 417A preamps, a popular design of the 90s, are likely putting out a signal on ~150mHz! I had to go to extraordinary lengths to get mine stable. He was a great teacher. That experience and education is why I worry about D3as going wild. Not all builders have the test gear to know for sure. I used the Bottlehead C4S a number of times but I was too "vintage crazed" back in the day, wouldn't use anything thought up after 1948. Now I am a lot more open-minded and even have a bag of Ixys current sources on my desk right now! Alas, I am too lazy to crank out builds and compare stuff like I used to. Putting a new spin on well-worn circuits as Shindo does, and Kondo did in the M7, using tubes in odd operating conditions, is a Japanese artform that appeals to my vintage side. I suppose it can sound good too. The linked article talks about the Shindo circuit sounding almost too nice. Erring in that direction might not be such a bad thing.