Hi raz_la,
First, use the Marantz's setup menu to make sure you have the center speaker set to "Small"--the VP100 IS a small speaker--so you do not want significant bass output going to it.
All the bass should be handled by your subwoofer and/or M60s if you don't have a subwoofer. For that kind of listening distance--15 feet--I'd prefer you use a larger center like the VP150.
However, if there is an intelligibility problem, where do you have the center mounted? If it's buried in a bookshelf or recessed in any way, boundary effects and reflections will degrade clarity. Move it so its front baffle protrudes slightly past a shelf or the top of the TV, or consider wall-mounting it with a Full Metal Bracked so you can tilt it towards the listening area.
Have you tilted the center down or up towards your listening area? Check the drivers to make certain there is no mechanical vibration that might be caused by a loose mounting screw. Hand-tighten the screws (don't overtighten) so there is no chance of mechanical vibration.
What is the Marantz's rated output into the center? Be sure you have any impedance switches on it set to 8 ohms.
Center channel speakers are not designed to produce significant bass output so don't set them to Large. I think that may be your problem.
