... simply ground loop issue ... if it only was that simple
You may have a "pin 1 problem". There is plenty of information about it, it even has its own website.
All your consumer gear which is connected to the preamp should not be connected to mains ground. It will float at some potential. Cinch cable shield should pull all of these boxes to the same potential. A thick shield has a small resistance, which is good here.
Disconnect the preamp and all the gear connected to it from the crossover. Connect the crossover with open inputs to the amps and see what happens. What happens when you short-circuit the crossover's inputs? You should get the pro gear (crossover and amps) clean (noise-free).
From here on it depends on your particular setup. Do some educated experiments and let us know.
Ruediger