I would tend to agree with Browning.

Every movie is mixed differently, there are flat out certain videos which are not going to sound as good as others. I would also agree that you may want to try using the receiver in it's normal surround sound modes like Dolby Digital, or DTS Masters as opposed to your Drama and Adventure modes. Those usually tend to add things like reverb, time delay, hal effects, etc...and I imagine are heavily influencing the quality of sound  you are getting from your center channel.

Also, did you run any sort of auto setup on your receiver when you first plugged thigns in? More often then not (which is why we are against using it) auto setup can add some very undesirable EQ to your speakers. It can make them sound a bit thin and hollow. We actually get calls often enough on this issue. If you have run any auto setup on the speakers, go in and disable it and make sure your speakers are EQ'd flat. This should help quite a bit. 

Hope this information helps. Please post your results or call us for more information. Our 5C is our most popular center channel for good  reason...