Yes. Blind 1 was that Jan had not told me about ff or what it could be. He just said here is something would you mind trying it out. So I knew nothing. As a matter of fact, it could have been that Jan wanted to show me how easily someone can hear difference alltough a switch does nothing. With...
You would need to ask Jan about why it is hard to be measured. I remember some discussions with him at the time he developed the idea and later when he had the A/B comparisons. I guess sometimes it is hard to measure subtle differences. But ask Jan. He is extremely open - yes he is Dutch ;-)...
To my knowledge ff is hard to measure. I did some blind testing with ff versus conventional and I was able to easily pick the ff. It has much more transparency. In my opinion ff is a big step forward.
The Soul sounds amazing. I own one of the prototypes which by the way was elected "best amp of the show" by Headfonia on the CanJam Europe event in Berlin. After additional modifications the SOUL was elected "best amplifier of the show" by the visitors of the Audiovista-show in Krefeld last year.