Tom's right, and the Denon ought to have the ability through the setup menu to switch each speaker on and off. While this is good for setting levels, it's even better for telling you if the center has become disconnected!
And that same setup menu will let you define your system as having no center speaker... so check that, too.
A good answer is easier with a clear question giving the make and model of everything. "The biggest problem in communication is the illusion that it has taken place." -- G. “Bernie” Shaw
