I've been having this issue for a long long time. Countless hours spent moving each source to a new outlet. Electricians looking to troubleshoot. Changing from HOSA cables to Mogami, etc.... I could have removed the mixer from the audio chain, but I wanted to still have some analog feel. So after researching I wanted to give this pyle PHE 400 a try. Mixed reviews on amazon, but wanted to give it a try. Glad I did, this solved the ground loop that I was getting from the Audio Interfaces. It did lower the volume a little bit, but they are reference monitors so I never really cranked them up to the volume I did in this video. Thanks for watching and hopefully this can solve your issue if you are facing the same interference.
Amazon Link: amzn.to/34BD8BV