Some Denon models have individual level controls for each input. I don't remember where that is on the setup menu, but you should be able to poke around and find it. If that had been changed while on demo, that could easily be the problem.Anyone...
Have you tried a microprocessor reset? That could fix the problem also, but it will wipe out any settings you have, including Audyssey - you will have to re-run it.