Hi blkwrwgn (that's a mouthful) and fuzztone (much easier).
I get a lot of my advice from the British Audiophile youtube channel - highly recommended. He has two videos, one on the Denafrips Iris (www.youtube.com/watch?v=3tKWWs-OMxw) that does a great job of explaining all the differences between digital outputs and why USB is better than coaxial, and I2S supposedly better than USB; and another video on upgrading your streamer/dac with an external power supply (www.youtube.com/watch?v=UmiLQ9ifb0A)
As I mention, if you want the I2S output and your DAC accepts I2S, then a device like the Denafrips Iris should be a cost-effective upgrade that still puts the price of the Zen Stream/Power supply/Digital to Digital package well below (comparable?) streamers from Auralic and Aurender and others.
As for external power supplies, the British Audiophile says he compared an iFi power supply with a linear power supply from a company called McRU. He found the LPS to be a significant improvement over the iFi power supply, though the iFi power supply was certainly an improvement over your standard cheapo wall plug (switch mode power supply).