WiiM into Outboard DAC

Yes :). I have a Wiim Ultra and while the built-in DAC is decent the external DAC is use is much better.  I use a MHDT Stockholm v2.

Would the Pro and Pro Plus sound the same as the Ultra if run through the MHDT ? Would the 3 sound identical, as a streamer, plugged into the same DAC?

@jl35 yes. as the Wiim will not be doing any conversion so the DAC being the same means the same data transfer to the DAC stage

I have all three units. 

From a potentially audible standpoint, the one argument for the Ultra is that it has USB output to an external DAC (along with optical & coax) while the Pro and Pro Plus devices only support optical & coax. While I don't find any difference between the outputs, there are others who have a strong preference one way or the other. 

Which one I use in what system ties more to features.  I have the Ultra in my main system and like the screen and extra inputs. I use the Pro in a headphone system where the amp has its own DAC. And I use the Pro Plus in a second system where a touch screen wouldn't be visible or easily used. 

Your mileage may vary.