There's a second thing that Audyssey does in addition to making each of the speakers equal in relation to each other, is to make -0 dB "reference" for movie playback. Changing the speaker levels, even in the same amount will move 0 from reference.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris