Apparently Ethan put a picture of his Null Tester on the cover of his 2nd Edition book.
However it's still in progress.
View attachment 19677
So Carver did something like this to prove he could voice one of his amps Identical to a very expensive tube design - and he succeeded. I want to say it was a challenge w J Gordon Holt? Possibly stereo review? Can’t recall the exact test but it was a null test in essence I believe.