Sorry for digging this up but I'm facing the same issue here and it's not the monitor cables, since the noise is also there on headphones. The noise is really fucking up my reamps. Any solutions? Everything (iMac, amps) is in one outlet.
It's to do with the iMac. I eventually cut pin one on the XLR cables(to my monitors) and it went away. Do the same to the cable you're using to your re-amp box. Worked for me.
