Keep in mind that a center speaker will not likely sound the same as your main speakers with pink noise.... that doesn't mean that they won't match well with real-world sources however. If you were to use the identical center speaker as your mains, it would likely be a bit better, but even then they may not match when using pink noise due to different room positioning!
My guess is that the center sounds a little "chesty" when mounted low because you're getting a bit of "boundary effect" from the floor.
Although your center in the "above" position might sound a bit thin when playing pink noise, my guess is that it's still a superior position in terms of dialog clarity.
I don't have experience with dual centers, though there has been a reasonable amount of discussion here of late that can be found with the search tool....
