I'm using the Quad L for my center, which is a direct match to my mains. For surrounds, I'm using a pair of Wharfedale bipole speakers, which have the same lineage as the Quads (Quad and Wharefdale are owned by the same parent company.) So the sonic signature is pretty consistent throughout the surround envelope.

To that point, there is a school of thought that all the speakers be matched so that the sonic characteristics are similar and there is some unifmormity in the sound across the entire surround filed.

In my experience, I think the most critical match is first and foremost between the mains and center -- so the entire front soundstage is well integrated. From there, some mixing and matching of the surrounds / rear surrounds is not a big deal. And certainly not an issue for the subwoofer.