Interesting discussion here, Alleric.
I will admit I've never tried low crossover settings (~40 Hz or lower) or run the mains full-range with the sub filling in below. Perhaps I'm a traditionalist, but I have always approached subwoofer integration from the notion of compromise. E.g. by using a subwoofer, I can gain authority and control with low-frequency content, but I must compromise on (possible) localization and "speed" of the woofer compared to the mains.
Since discovering Rythmik Audio servo subwoofers, my notion of compromise is nearly irrelevant. I have had great success integrating the sub/mains and it's difficult to pick out the sub now. I can choose just about any crossover point (within reason) and get superb integration. I believe that is a testament of the quality for both the Sierra-1 speakers and the Rythmik F15.
With that said, I usually run full-range for most music and run the sub for most movies (80 Hz for mains/rears and 60 Hz for center). When I'm in the mood for loud and lively (> 85 dBC avg), I'll use the sub for both music and movies to ease the load on the amp/speaker.
Anyway, you got me curious here, so I think I'll play around with the very low crossover options this weekend, as well as "fill in" mode.