WiiM into Outboard DAC

It may depend on the engineering of the outboard DAC.

A great DAC, with good FIFO, isolation of the input(s) to reduce noise, and a top quality clock (OCXO) is effectively insensitive to the source (given reasonable competence of the source).

In the early days of digital, DACs were very sensitive to their source as they reconstructed the  clock from the input stream and had poor isolation, so different CD transports and different digital cables seriously affected the DAC's sound. Jitter was a major problem. In my experience today's better DACs do not have that issue.

One thing to look at is the device's output capabilities and the DAC's input specs.  There may be limitations on DSD inputs for example.  My Esoteric DAC only supports DSD 512 over USB.

There may be differences in the streamer's noise rejection from the input CAT-5/6 - look for reviews on that topic.

if you’re using ONLY the digital outs into an external DAC, there’s basically no real sonic difference between WiiM Pro and Pro Plus, and only a maybe, sorta, tiny one with Ultra via USB. Any changes are going to be way smaller than what your DAC/room/speakers are already doing.

@misstl Sorry, I had the Ultra. 

Thanks for all the responses...A friend is giving me a used Pro Plus, which seems the sweet spot to use as stand alone or with DAC, as I do not need the features of the Ultra, and this will be in a secondary system...