Sorry if this sounds like a dumb question, but just need to make sure you are not missing the pre/main jumpers. When you tried the preamp with another amp, did you have to physically remove a jumper between pre out and main in on the 7140 like the ones shown in this picture.
http://img.canuckaudiomart.com/uploads/large/182730-nad_7140_receiver.jpg
Those jumpers are required for the 7140 if it is only used with speakers and a source.
http://img.canuckaudiomart.com/uploads/large/182730-nad_7140_receiver.jpg
Those jumpers are required for the 7140 if it is only used with speakers and a source.