The key for me was finding a center channel that would fit in the 6 inch space between the mantle and the display and that would have a total depth (with speaker cables connected) of 5 or less inches (i.e. the depth of the mantle).
Given the size restrictions, I had fairly limited choices and went with the Canton CD series (CD100 center channel and CD50 L/R). Very solid construction, clear dialogue, fairly detailed with some high frequency roll off and a sound that belies the size (although the center channel is actually not that small). Not what I would choose for a main HT setup, but more than good enough for my bedroom rig.
For my other rig, I have a Martin Logan Fresco center channel, which has the same depth as my display, looks terrific mounted flush against the bottom of the display, and sounds fantastic. At 8.25"H, however, it might not be the best choice for cc that has to go between a display and a mantle. The Vignette is smaller, but only by an inch in that dimension.