Best tube dac available?

You've got three heavy hitters right off the bat! :-)

i believe audio research makes a tube dac as well as amr. i am not sure of the model names.

regarding the zanden. i prefer the original zanden to the later versions. i find the original version more full bodied and more in keeping with the classic tube sound, but not to the extent of sounding syrupy , veiled or a significant loss of resolution.

Also worth checking out the Ayon Skylla. I have been using the CD5S which is the Skylla + CDPro2M

Just announced at CES: Audio Research Reference DAC which has the analog stages from a Reference 5 SE (itself using parts derived from the 40th Anniversary Edition). The Reference DAC is also a network connected media player, along with USB/SPDIF/AES inputs.