Ok, My 2 cents worth.
You've just discovered an audio truth: All systems/components need individual calibration. Even between channels on the same amplifier you're going to find voltage differences between channels. Electronics manufacturing is far from an exact science. The distances, resistances, capacitances, etc. are going to have variations in different signal paths within a piece of gear, let alone two separate pieces of gear. Then you add even more organic materials into the mix (i.e. speakers-wood, fiberglass, etc.) and your differences become even more pronounced and measurable. Of course your speakers are going to have different sensitivities. You are using very different speakers for every position! (By the way, I love the Carver ribbons. There's rumor that BC is going to design more.) The center speaker is of completely different design than your fronts. I would have been extremely surprised if they had measured the same.
Bottom line: All systems sound better when completely calibrated and dialed-in. And they all measure and respond differently.
Big Shock!
Mix.