Have you tried connecting the Marantz to the Amp without the crossover? It could also be a bad cable. XLR cables are pretty tough but they can have issues. It could also be a bad connector socket. Try connecting one signal cable at a time (power off) and see if one produces the noise. You could also try the RCA input for signal connect. Is your sub powered from the Furman too? Can you ground the Marantz to the Furman with a piece of speaker wire just to check if the noise goes away (the Furman might have a lug for just that purpose). If you have already tried all this I would steer toward the crossover being the issue and then the Marantz and then the Furman since the amp has no noise with no input.