Center Channel Dialogue

As others have said, something is wrong here whether it be a setting in the processor, defect in the processor, defect in the speaker itself, speaker connection, speaker setting for how the center content is handled, etc. The center channel carries MOST of the sound and dialog for most everything that has to do with AV sound during a movie or TV show. Try putting ANY speaker in place of the center and see if the speaker itself may be defective. That's an easy thing to troubleshoot, and then work backward to the processor. 

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.

I'm not familiar with your equipment, but I am familiar with the problem.
My center channel is on a cabinet under my TV about a foot from the wall.  The first thing I added were some isolation feet to the fronts.  I noticed cleaner bass, so I added them to the center and surrounds.  Not sure if they did anything for the center and surrounds, but maybe.  The next thing I added was 3 layers of carpet pad foam cover with cloth under my center extending over the back of the cabinet.  That helped a lot, but....  My Denon AVR was getting long in the tooth and was replaced with an Anthem processor.  Great improvement!  I think the Anthem room correction helped with the bass in the room and made everything sound better.  Audyssey didn't work well with my Golden Ear speakers with built in subs.  Great center channel clarity at higher volumes. The final solution was hearing aides....

You didn't mention anything about your acoustical treatments, which are the #1 issue when it comes to unintelligible dialogue.

I agree that room acoustics are a major part of good sound and the OP should probably focus on that first. He didn't mention anything about his room and maybe I I incorrectly took that for granted..
I have an irregular shaped open room, vaulted ceilings, and tiled floors.  I added some absorptive panels, shag throw rugs with padding, and a crude bass trap. It made my room better, but still not ideal.  I forgot to mention I did all of that before the other things and still didn't have clear dialogue.  So, I guess the "first" thing I said I did wasn't really the first. 
I'm not sure if you can cost effectively "treat" an average room to the point of making any system sound good, but maybe?