If the switch needs to be set to provide either RF grounding or grounding down to DC, the most likely reason is that there is no ground/earth for the system on one or both sides of the iGalvanic 3.0.
In this case USB cables are in effect not shielded against ground/earth referenced noise sources and may therefore result in dropouts if interference is present.
Where one or both sides of a system have no ground/earth connection the earth connection should be restored, for example using iFi’s “Groundhog” or by switching the ground link switch on the iGalvanic 3.0 correctly.
