As other have suggested I would check:
1) The set-up parameters of the center channel in the Denon
2) Use a different input/channel of the Denon for the center to see if it a problem within the Denon specific to the center input you are using (e.g. bad amp channel)
3) If possible substitute a satellite speaker for the center speaker to see if the problem is the speaker itself
4) Also play music through all channels as another way to check the center speaker
5) Turn off the center channel in the panasonic so the dialogue is handled by the front L and R speakers
This all should help pinpoint the problem.
1) The set-up parameters of the center channel in the Denon
2) Use a different input/channel of the Denon for the center to see if it a problem within the Denon specific to the center input you are using (e.g. bad amp channel)
3) If possible substitute a satellite speaker for the center speaker to see if the problem is the speaker itself
4) Also play music through all channels as another way to check the center speaker
5) Turn off the center channel in the panasonic so the dialogue is handled by the front L and R speakers
This all should help pinpoint the problem.