Center channel speakers are generally a problem solver, not a problem creator. This one is perplexing.
From my perspective:
1) Accurate, high resolution center sound is not leaving the preamp. This has not been my experience with Marantz surround processor/preamps, but could very well be the case here. A highly relevant question is what is your movie source? Your source component? Cabling? Power delivery? All of this matters. Try sending a (variable output) source you trust directly into the power amp and judge the quality of what you hearing at the speaker. This may tell you if the source and/or preamp are the issue.
2) The speaker has issues. As suggested by others, try a known speaker in place of the one your are using now.
3) The room. Try standing 2' away from the speaker at position your ears at speaker level at a moderate level. If the sound is good, something is happening between you and the speaker.
Some suggested to bypass the center speaker. This would be simple to do with the preamp settings and specifying the center channel to "none". This will move the channel channel information equally to left/right main speakers. You might like this better. It could make things worse in that the path from a single speaker to you is less complex. When adding mutilple speakers, you're doubling the complexity of the waveform's involvement with room boundaries, etc. If you're not located in the sweet spot,the speakers will have a phase/time delay and will blurr the image, making it less intelligible. Which is not the goal.
If you're seated some distance away, a horn-type speaker will have more controlled directivity resulting in less room interaction, which might be a good thing.
Good luck.