From BAT’s website for one model:
VK-90t also means there are no tubes to bias; no fuses to replace; no pentodes imitating triodes; and no direct current at the tube output stage. VK-90t is the sound of the future—and musical truth.
I don’t know if the no-DC is a feature of the circuit, or if they have a circuit that adjusts to prevent DC? I am sure there are some tube hacks that can weigh in.