To answer the question of what I'll be subjectively looking for in a DAC:
For this to be successful I would expect any DAC that measures well to provide quality at the minimum level of what my GG2 is doing now. I have valves that are pretty linear, as well as those that would be considered high distortion (typically described as warm) so I will compare against the most linear ones I have.
A very impressive soundstage
Wide soundstage with the speakers effectively disappearing
Deep (front to back) imaging
Height (top to bottom) imaging
I would expect instruments to be very precisely placed and no smearing of detail. A very high degree of clarity in the mix.
I would not expect a dull treble, lack of midrange or excessively bloated bass.
Feel free to modify the parameters above, these are first thoughts. In simple terms if measurement is what matters then the cheaper DAC should be superior.