Sorry I meant -20db.
Distortion can happen at many stages. At the mic capsule, the mic transmitter sensitivity , the receiver, or the recorders input electronics, (this case) or the recording level settings. All must be aligned optimally.
The clue to yours us that the level is not high and a flat line clip has taken place so I guess it’s the input stage.
Plus I had exactly same with a Zoom.
The -20 dbpads will bring a hot (pro level ) feed back a touch and stop it hitting the inputs too hard .
Mine were off eBay .
Peter
Post Production Dubbing Mixer