Like Videodrome, my first thought was the amp being overdriven and going into a protection mode. The digital connection isn't going to know whether the undecoded bits are a quiet scene or a loud scene, and even after being decoded it shouldn't matter either way to the processing. It's more likely happening in teh amp section. Speaker efficiency, crossover settings (particularly lowering them and thus placing more load on the internal amps), changes in room size (which goes back to efficiency to some degree since it affects the amount of power needed to achieve a certain SPL), or changes in listening habits could all contribute to this developing "out of the blue" rather than being present from day one.