Buzz originating from amp...please help ?

I bought a Pass X250 through Audiogon and when hooked up to my system, a hum or buzz is coming out of the tweeters and midrange speakers on both channels. After exhaustive trial and error, I find that the buzz does not exist when the amp is not hooked up to any other component. It is of course connected to the speakers. If I attach a pair of interconnects to the amp (whether or not these interconnects are connected to the preamp), the buzz comes back. Does anybody have any ideas what I should try, or what the problem may be? Thanks for any imput.