Eliminating Ground Loop Problem

I use a Marantz 7702 MKII processor, ATI 3-channel amplifier to drive my center and rear channels, and my Jeff Rowland Corus preamp with HT bypass when watching movies. I'm using RCA interconnect cables from the Marantz to the ATI, tried balanced XLR cables with no change. I contacted Marantz and they didn't provide any advice. ATI helped me troubleshoot the issue and we isolated the problem to the Marantz. Any advice on how to eliminate the ground loop issue?