I have Mission 774's and the new PSB Alpha C for my center. Now obviously, these speakers are not matched for each other. Is there any way to timbre match them as closely as possible?
Speakers are timbre matched by design. You cannot alter a speaker to make it match another. Timbre matching comes from the driver design, size, material, cabinet construction, and crossover design. This is why you want to at least get your front 3 speakers from the same line from the same manufacturer.
If the tweeter types are the same, they should at least sound similar (both are titanium, soft dome, etc....).