I don't have time to re-read all of what's been said already but you need to break it down to the least common denominator and start testing from there.

1. Does the Denon respond to it's own remote via line of site? If no, and you're sure the remote is good/fresh batteries...you have a bad AVR, time for service/exchange. If yes...

2. Does the Denon respond to it's own remote thru your IR distribution system? If no, try another source thru the IR distribution system such as a cable box with the original cable box remote. Still no, you have an issue with that system. If yes...

3. Does the Denon respond to commands from your Russound controller? If no, you have may have an issue with the Russound hardware/programming or via the cabling between the two to the emitter, improper emitter placement or a bad emitter. Send a command via Russound and look at the emitter with your cell camera. See a light blink? Then you have something happening as far as IR.

4. Does Denon respond to the Harmony via line of site? If no, you may have a bad Harmony or incorrect programming in the Harmony. If yes, does it work via the IR system with the Harmony? If no, go back to # 2 and troubleshoot the IR system with continuity tester and your cell camera to see if it's receiving IR at the emitter.

5. Hire a pro. Stuff like this could take you weeks to figure out...I think it already has, right? I bet anyone with a year or so under their belt could have you back in business fairly quickly. What's your time worth?