A very interesting system, Karel. I set out to achieve something similar in a second system, and I wound up with hand-built split-chassis 300B SET monoblocks driving Living Voice IBX-R2 speakers. I would guess that your system excels in the reproduction of the same style of music I targeted in putting together my second system: namely, jazz, vocals and classical chamber music.
I wonder how the sound of our two systems would compare. In my case, the tonal balance is quite neutral for an SET system. The frequency extension, speed and PRaT are far beyond what you would ordinarily expect. In addition, the system excels in the stereotypical areas of presence, image saturation and palpability with a slight but not exaggerated midrange warmth.
The Zu speakers have always interested me. I can't help but wonder how the sound of your system might compare with my benchmark in the realm of low-powered tube amplification and high-efficiency speakers...