EasyTarget, most receivers have the ability to control the volume of each channel. I suggest bumping up the center volume to help you hear dialog, but keep the front R L channel volume the same, so the loud parts of the show don't get too loud. I find the really loud parts are mostly the R&L channels.
As to lackluster sound, as GHD said, your receiver does not have power (current) to dynamically drive your R&L channels to their full potential. And the overall quality of the processing and output of the receiver will limit performance.