My suggestions:
1. Check polarity of the speakers. Electrons don't care what colour of insulation surrounds the wire they go through. Just make sure the colour on the pos. terminal of the receiver matches the pos terminal of speaker.
2. Check speaker volume levels. Most receivers have a tone generator. As long as the center channel speaker isn't grossly different from the mains, your ears should be able to tell if they are roughly the same.
3. If the amp has eq settings for each speaker, make sure they aren't causing an issue. Try a flat/defeat setting and see what happens.
4. Check the center channel speaker by covering the tweeter with your hand. If the highs don't change, then you may have a blown tweeter. This is easier to tell if you can select some sort of "All-channel stereo" mode where the receiver mixes the left and right channels into the center, as Dolby tends to "steer" the audio. Failing that, find a video source with lots of center channel dialogue. I recently experienced this failure. Maybe this is your problem too. I'm not familiar with the Denon receiver, so you will have to consult the manual.