I am not a technician but today I discovered this thing :
if you connect a XLR with external unpainted metal chassy the noise problem disappear; on the contrary, if it is painted or made of plastic, the noise is present.
This happens because inside the XLR female socket of the Quad Cortex there is a metal tab that ground the XLR cable. If the tab comes into contact with the outline of the XLR connector it interrupts the ground loop.
So the solution to this problem is to use XLR cables with an unpainted metal outer chassy.
