Sorry for all the inconveniences you experienced with our products and thank you for reporting the issues you experienced.
Thanks to all the videos you provided, engineers were able to see how those units are used and connected when the issue occurred. It was a great help for the engineers to identify the root cause of the issues. The answer below may apply not only to INTERFACE 2 but also to other mixers including DJM-250MK2.
This is the official answer from INTERFACE 2 engineers.
"We designed the INTERFACE2 to have the optimum sound quality when combined with a turntable that's equipped with a ground cable, such as the PLX-1000 etc.
Turntables without a ground cable, for example, the PLX-500 etc., tends to have an unstable ground connection.
The sound might appear better either when using the phono preamp on the turntable, or when using the phono preamp on the mixer, depending on the ground's situation.
Since the sound quality differs depending on the usage and setup, please audit the actual sound and use the preferred setup."
If you have any questions or issues, please let us know.
Thank you for your kind understanding.
CRM, Pioneer DJ