It might help to know more about your setup. Are any of the connections normalled? What patchbay? In the spirit of troubleshooting, did any settings change on the interface during the setup process? Are you using TRS connections from the interface? Are you sure all cables are fully seated?
This might be better suited for the Questions category.
As another data point, I’m using a patchbay with multiple audio interfaces, a half-dozen synths, modular, and outboard and have never had level/distortion issues. All issues were usually traced to a cable coming loose or rarely, just a bad cable.