The Zendrive 2 is a big box version of the Zendrive, using a 12AX7 internally. Sadly the tube is only being used for clipping, replacing the BAT41 and 1N34a diodes in the small box Zendrive. But according to reliable sources, it sound very good, with some harmonic mojo happening. Other than the tube clipping, there's only a few value changes from the small box version. Schematic for reference.