What does audyssey do where subwoofer rolls off

I'm not completely certain, but.....

I believe that Audyssey behaves exactly as you describe with the EQ lower limit set at 20hz. The set-up sweeps are run at a modest SPL (75db), so most high quality powered subs shouldn't misbehave too badly during set-up. I'd guess that any real overdrive problems on most quality subs would only become evident at higher SPLs. As long as you turn up the volume dial slowly after set-up, I think you'll avoid any really unpleasant surprises (damaged gear).

Remember, too, that the ear isnt terribly sensitive to high distortion levels at very low frequencies and that most music has very little energy way down there. A little distortion during the car explosion sequence probably won't be enough to distract you. OTOH, if you listen to pipe organ music at 110 + db and you want to play it safe, get two (or more) high output subs for an Audyssey equipped system. I use two 12" Rythmik subs and my sense is that I'm not really near max clean SPL in my loud listening sessions even with deep bass heavy music material.

As a practical matter, I think one quality sub should be okay in normal sized rooms with normal program material at normal SPLs.

While some EQ systems have been developed to tame peaks only, my understanding (again, not certain, but vaguely recalled from days researching the system) is that Audyssey actually does boost nulls, as well. It may well be that any boost is limited for frequency and/or amplitude to avoid the "runaway amp draw" being discussed here, but I do seem to recall both boost and cut. BTW, certain other EQ systems (Velodyne for one) definitely include boost, so Audyssey would not be unique in this regard.

Again, I can't claim certainty on this point, but I did a boatload of research on Audyssey 4 or 5 years ago and I remember it this way. Then again, my memory ain't what it used to be!