my favorite 12au7 is the brimar with the yellow lettering.
regarding tube dacs, again.
as has been said many times there is no best of any component.
however, there are two basic approaches for designing tube components. the classic tube sound, of which the zanden is an example, or the concert fidelity,as rhe exemplar of the modern tube sound. i suppose that altering the tubes, affects the sound, but the approaches represent what the designer wants to achieve.
i also like wavelength audio non usb dacs, e.g., an early cosecant.