The F-M curve may partly explain the sense of more "air", but based on articles I've read, and some personal conversations that I've had with Richard Vandersteen, there is a more likely explanation. Using a subwoofer with crossover that handles the lower two octaves relieves the main speakers from handling the lower frequencies, thereby reducing distortion from the main drivers.
When I installed my pair of Vandy 2Wq subs, one of the first things I noticed -- after better defined and deeper bass -- was greater overall transparency, dynamics, and sound-staging from the main speakers (Vandy 3A Sig's). And I know it was not just my impression -- several friends who have "good ears" also heard the overall improvement in the sound quality of the main speakers.
When I installed my pair of Vandy 2Wq subs, one of the first things I noticed -- after better defined and deeper bass -- was greater overall transparency, dynamics, and sound-staging from the main speakers (Vandy 3A Sig's). And I know it was not just my impression -- several friends who have "good ears" also heard the overall improvement in the sound quality of the main speakers.