As some of you have noticed with the Mystery Amp, and now possibly the coming TORII JR., I am doing work with Ultra-Linear transformers despite bashing them in my original design papers for the original TORII. Since ultra-linear is a form of feedback my purist tunnel vision of 20 years ago steered me away from it.
I had a uniquely good listening room in the 90's that is responsible for me starting Decware. I spent 10 years listening in it every night from 10:00 P.M. to 3:00 A.M. and used that time to form my opinions on how things should sound. (single, self employed and no neighbors made such a thing possible)
It was actually four rooms that were interconnected together with large openings. Below is an illustration of the room.

I had a visual line of sight that extended 30 feet behind the speakers. I became addicted to sound stage depth.
I found typical use of negative feedback destroys sound stage depth, preventing it from reaching the pink zone above. The difference between Ultra-linear and non-ultra-linear was making small differences in the pink zone. This was 20 years ago and my amps have evolved considerably since then and I have also realized that in 20 years, 10,000 customers, 54 countries, only 6 people have a room that even comes close to the above illustration, therefor the pink zone will never be experienced by 99.9% of our customers. This is why after the great success of my Zen Mystery Amp, I have been revisiting ultra-linear designs.
Steve