"What I can tell you though is that typically the center channel is something like 13db down from the LR fronts."
Hopefully, this is a typo. There is NO way the center channel is 13db down from the mains. So, I'm gonna assume you meant to type "3db down".
Even so, Mtrot is absolutely correct in stating that the calibration will equalize the output between the mains and the center.
And you are correct that the center channel is not tasked with supplying much bass content. Its main job is to anchor the voice track to the center of the soundstage. Nonetheless, you definitely want a center channel speaker that is voiced as closely as possible to the mains from approx. 60 Hz on up. This will aid in maintaining a seamless image across the front of the soundstage.
Also, having properly voiced rear channel speakers helps to provide a seamless soundfield when sounds are panned from left to right and from front to back.
-RW-