Classe' preamp ground loop hum. Help!

I just bought a Classe CP-60 preamp. When I hooked it up i got a vicious ground loop hum. I mean a wicked hum, like if I was running cheap speakers they would be toast. Checked all outlets on circuit, perfect ground. Disconnected everything but amp, preamp and source. Still ground loop hum. Hooked up Sony TAe9000ES in place of Classe', ground loop hum goes away. Do i just have a defective preamp? Suggestions? Advice?