Had a look at the 8NU8R specs. Is the hum occuring when you have your output plugged in, but no input into the 8NU8R? If so, then what you're running into is normal for the module, since it's designed to output an offset DC voltage when no input is present. Headphones might not pick up on the humming, but with something active such as a preamp stage (as in the Focusrite box), it's going to be brought up to audio levels...and then some. Plus, running DC offset directly into a preamp, depending on the type of preamp, might not be a good idea.
Really, that module isn't designed to be a proper output, upon closer examination. A proper output is something more like this: https://www.modulargrid.net/e/pittsburgh-modular-lifeforms-outs where you have a ganged stereo attenuator and 1/4" outs, possibly also a separate headphone amp, and the like. While the point about the star ground I made earlier is also valid here, using a proper output module is probably what's actually needed.