The signal path is:Serato DJ -> internal trim/volume per deck -> Serato DJ master volume -> USB (set to 0db in the DJM V10 Utility -> DJM V10 -> Channel 1 Trim -> Channel 1 Fader -> Master output -> Speakers
After checking that whole signal path the only thing that eliminates the distortion is at the Serato DJ Internal trim/volume setting.
This is control vinyl. So it's not like the volume of the timecode signal going into the Phono jacks on Channel 1 are going to contribute to clipping anything.