I realize these posts are rather old, but I thought I'd share some knowledge I have of the processing in the Denon/Marantz products.
My direct experience is with the Denon 3313CI (a 2013 product), but AFAICT, most recent Denon and Marantz products are similar.
The short of it is that for the best performance I recommend setting up the system so that the sub level in the AVR is as low as possible without going below the limit of "-12". Practically speaking, aiming for "-10" (at the most) is probably completely reasonable, and even much higher settings will be OK "most of the time". See below.
In more detail: With Audyssey off (*), the AVR appears to be designed so that a hypothetical 7.1 channel soundtrack in which sub bass is playing at maximum level ("digital full-scale") on all of the channels at the same time will be output cleanly by the AVR if the sum of the master volume and sub level are less than -12. If either the master volume or sub level add up to to a number greater than -12, then it is *theoretically possible* for the bass from a 7.1 channel soundtrack to overload the subwoofer output. So for example, if you don't ever listen higher than "-6" on the master volume, then a sub level of "-6" would be OK, but you need "-12" to not overload if you sometimes turn up to "0".
(*) BUT! If Audyssey is running, things get a lot more complicated and overload is possible at *much lower levels*. Audyssey can boost the signal quite a bit. How much precisely is very complicated, especially if Dynamic EQ is enabled. In other words, it's a good idea to setup the system with the sub level as low as possible if Audyssey is being used, even if the volume is kept fairly moderate. That way you get as much capability out of the subs as possible before they overload, and when they overload, they will do so much more gracefully than when the AVR output overloads.
As far as actual movie content is concerned, I'm not aware of any soundtracks that play bass at maximum ("digital full-scale") level on all channels at the same time, but a few get within a couple dB. Lots more get within 6 dB of the max.
I can't speak for other AVR/processor manufacturer except to say that every product has limits and some designs are more flexible about getting around those limits than others. Unfortunately, information about this sort of thing is scarce, even at places like AVSForum. I figured out the limits of my Denon with careful testing and found corroboration for my findings in other products elsewhere. It's a shame these details aren't well documented by the manufacturers because they are crucial to optimizing a high performance system.
Trust me, this is a bigger deal than most people realize. I bet a lot of sub capability goes to waste simply because people are overloading in an AVR or processor.