Just a quick note. My brother was noticing the same phenomena as you. He ended up having his hearing checked and discovered that he had developed a loss in the frequency range exactly corresponding to the vocal range. As it was slight, no need for an aid, he simply adjusted the volume to make up for his loss.
One thing that is almost never mentioned in guides to set up home audio, is our personal sensitivity to frequency ranges. The whole point of home A/V is to create a pleasurable experience. If you are happier with the level of the center speaker higher, then go for it. There are so many variables involved in setting to reference levels that I wonder if we shouldn't look at the process almost as a good cook looks at a recipe. Start with the basics, but feel free to make adjustments based on personal taste.
