I learned from a friend, and XTI4000, owner of a problem with a ribbon connector inside the amps. He was given to understand there was a run of Xti's that had a bad ribbon connector. His amp, as others had experienced, would be on, but no sound. Upon opening the case light tapping on this ribbon connector would reveal an intermittent signal. simply easing out the connector and pushing it back in a few times cleared the problem. They were evidently not seated well in the manufacturing, or possibly some goo or other material is causing a break in good contact with one or more of the pins in this ribbon connector.
I checked both my brand new amps and found one had this problem. Since "cleaning" the contacts I've not had a problem.