hmm... I remember reading about a tube DAC as some sort of DYI project on a home audio forum...which I think would be an awfully cool place to have one: in the home stereo. If the DAC were built right, I bet it would sound fantastic. I know....that was a lot of help wasn't it?
I don't know that I'd want one to mix with though. I mix with a set of ASP8's and it's entirely frustrating to nudge & polish a mix into sounding fantastic, then load it up in the car & realize that the ASP8's have done a grand job of making everything sound better than it really is. Liars they are!!
I would imagine you would wind up with a similar situation with a tube DAC in your monitor chain. Maybe not quite as dramatic as the situation I have, but there's a good chance of having some 'fake flattery' going on none the less.
Of course, you could always get your mix where you're going on the tubes, then record the DAC back into the DAW & boost that out as your final mix...that would get the tube mojo into the digital world where everyone could hear it..not just you.
Anyway...that's my $.02..