Murph,
As I mentioned before, I always leave that DTS setting alone. The only one that I sometimes change is Cinema Compensation. This adjusts the treble. (Most?) movies are recorded with a treble emphasis to compensate for high frequency attenuation due to the movie screen. If the treble in a movie sounds too harsh, I enable this and voila...perfect treble.
But let's get to your issue. My 600 is certainly not infallible (sorry Axiom). No sub is. But the only way I've discovered its vulnerability is by pumping very low frequency (below 22Hz in my case) pure tones at high SPL levels (over 100dB). With these pure tones and levels, I can definitely hear distortion similar to what your describe. Rattling is perhaps a harsh term to describe what I hear. Port noise, chuffing and a "tick-tick-tick" sound is a better description. But I have never been able to detect this during music and movie listening even when I turn it up stupidly loud.
Regarding the fluctuating dB levels, do you have your meter set to slow? When I set mine to slow, I don't have that problem but with fast I certainly do.
