And I just read Rob's explanation on the single active gain stage. This follow up deserves it's own post.
All DACs have a gain stage, Hugo's is simpler and better implemented. That same gain stage can also power your headphones. So no, it doesn't have a separate amp stage on it, you're listening to a DAC with a revolutionary gain stage on it.
That means the technically best sound you can get out of the Hugo is to not attach anything and listen straight from Hugo. Some headphones might require more power from an external amp, but it's still adding a boat-load to the signal path. But sometimes that suits preferences better. For me, Cavalli's designs are also very simple, pure, and elegantly implemented, and the added power out of the Liquid Gold makes the Abyss sing.