On Hugo 2 optical sounds the same as USB - and this is via headphones, so no ground loop from the source, and hence no ground currents. It changes as soon as you ground the connection, which normally happens with Qutest, as you will connect an amp on the outputs. Now to reduce the ground currents (RF noise into the ground plane of the DAC) I use galvanic isolation and RF filters - but it's not possible to totally eliminate the issue. If you use a low power, simple source, then USB sounds the same as optical (particularly with battery powered sources), so use the optical as your benchmark. To do the listening tests correctly you need to disconnect all other sources. Coax has no galvanic isolation, and direct coupling to the grounds, so will always be the brighter sounding input.
The reason it sounds brighter is RF noise when interfering with analogue creates more noise floor modulation, which makes it sound brighter.