My Innuos Zenith SE only outputs usb as is the case for their top of the range £10k Zenith Statement which is completely designed around being low noise and also only has USB out. When I asked them about why they didn’t use optical, bearing in mind its RF isolating aspects, they said it was not that simple. For instance, if their usb output was already RF free due to their design within the Zenith then it could outperform optical for noise at the ultimate end of things because the optical converter inside the dac can itself generate electrical noise into the converted signal within the dac. They came to this conclusion have measured both options. So, whilst in general terms optical is better than usb for RF noise it is perhaps possible that those of us who use usb might actually have the ultimate system interface if our usb source is already noise free. Obviously there are copious amounts of YMMV in this.