The Aurender has a full-featured, advanced DAC internally, and I imagine the differences could be small between it and the Yggy. I've personally done group comparisons with the Yggy and other DACs and do not doubt what my ears and their ears found to be different.
It also seems like the Aurender, while offering the option, is designed as a Reference DAC and streamer, and the advantages of signal path and proximity certainly favor the Aurender's DAC. Perhaps if you had an EMM labs DAC you would utilize the option, but purchasing an Yggy in addition to the Aurender does not make much sense to me.
Good to know Aurender has a pretty nice DAC in that thing.