Check if the center speaker's distance is set to a somewhat higher number (that is, a greater distance, as it often should be) than the front left and right speakers. If all three of your front speakers are in the same wall and are set to the same distance, you will hear the center speaker a moment sooner than the front mains because it is closer to the center seat. It can sometimes be noticeable. A tape measure is helpful.
I also have had the same issue as you described. After using a sound-level meter to balance levels of the three front speakers using the AVR's test tone, I reduced the center speaker's level manually a bit at a time by ear
. You can do it totally by ear, too. It did not take long to find a level that retained dialog clarity and allowed the center speaker to blend nicely with the front left and right speakers (see page 221 in the Marantz manual, linked below).
Also, check the tonality of the three front speakers with the test tone, listening carefully. You might find that the center speaker is brighter or more recessed in the upper midrange and/or treble, for example, making that speaker audibly stand out. Use the AVR's manual EQ (page 178) to more closely match its tonality with the front mains.
Finally, your Marantz has a feature called Dialog Control (page 168) that might help as a last resort.
PS: When it is only the two of us (wife and moi) in the theater room, sitting in the middle, we prefer the phantom mode.