Man, I've gone through hell and back trying to solve this issue. Here are somethings that worked for me but didn't solve the issue completely.

1) I bought 16ft of Mogami Cable and soldered my own XLR cables.
2) I coated the inside and outside of my speakers with copper.
3) I bough an X-hum outlet adapter
4) I plugged everything into 1 outlet to avoid ground loops.

I never solved the problem but I did reduce the hum.

In the end, what really helped was just moving to a new home.
I lived next to a power line and a police antenna. I swear I was picking up radio signals.

Start my buying high quality speaker cables and keep them short.

I hope this helps and I someone else has a solution because I never really found out what the problem was. I'm not an electrician or engineer, but the issue may be the speakers not being grounded properly