It was recently brought to my attention that Zanden's Flagship Digital front end has a tube based DAC. Are there other SOTA digital front ends that have tube based DAC's?
AN DAC 5 Special and Signature
I know we can do better than one response:-) I would include tube based CD players in the discussion. What about Audio Research CD8, Abbington Research CD77(sp), wavelength products? Do any of these represent state of the art in digital playback?
This is amongst the best out there

Review: Accustic Arts Reference Tube Hybrid DAC II DA converter

You can also match with its companion reference2 transport
Accustic Art Reference Drive2
Bflowers, you should take a look at the APL Hi-Fi NWO-4.0-SE if your looking for state of the art.
As far as one box players are concerned, both the AMR CD-77 and ARC CD8 are outstanding and I would add Einstein "The Source" to those to complete my personal Tubed SoTA Triumvirate.

As far as tubed stand-alone DACs are concerned, keep an eye on the Pink Faun DAC to be released soon. If it's anything like their tube amps this promises to be either great for the money or simply unrestrictedly great.
I'll throw in another DAC for the sake of discussion, since I have regrettably not had the pleasure to hear this myself until now. How about the Jadis JS1 MkIII DAC?