help integrating preamp into HT making loud buzz

Newbie needs help making the connections to integrate a VTL 2.5 tube pre-amp into my Rotel 1066 suround sound processor system. (Rotel 1075 5-channel amp / B&W speakers). I can hook up all the power cords, and have no noise. But as soon as I hook up interconects from the two seperate components to the single ampifier, I get a very loud buzz/hum through the speakers. Hooking either the processor or the preamp to the amplifier is not a problem, but not both at the same time. I'm running my disc player to the VTL, the VTL preouts to the front channel inputs on the amp, and the preouts from the rotel into the rear / center inputs on the amp.

I can still hear music though the speakers, but the buzz is much louder than the signal.

I also had the same problem using a Cary pre-amp.
please help.