@Paul McNeil , just to be clear, many of us do feel the MultEQ X editor app, or the better MultEQ X app can in fact make a "significant improvement" (borrow your own words), but that's based on measurements such as using REW and the Umik-1 mic.
Keep in mind, as
@AcuDefTechGuy (who has a ton of experience with Denon and Yamaha systems) said, better, best measurements and specification may not reflect what your ears/brains prefer. So it is possible that you may not prefer the improvements on paper that the 8805 with Audyssey+App can give you.
For the 15-200 Hz range though, ACDT may be one of the outlier, as most Audyssey users (based on various forum posts) do feel the bass (with some minor tweak at least) are audibly tighter and smoother though many feel Dirac Live's can do even better. Again, many seemed to prefer using no RC/EQ at all, or do their manual tweaks such as using PEQs offer by YPAO, REW, etc.