I would be less concerned about the rear channels than the mismatch between your fronts and center channels. The "logical" change would be to replace the C-30 with either another Gem or a Voice from the same top tier Ultima line. The C-30 while nice, is from the entry level Concerta line. As the center channel is arguably the most important-and should tonally blend seamlessly with the fronts, this is the place to start IMHO.
