It's actually 8,66 inches (220mm).
Do you mean that Audyssey MultEQ XT32 can and will do the same thing that the ATM is doing?
I cannot imagine it does. That would basically mean that Audyssey is a danger to all speakers and subs that natively won't go down to 20Hz.
I thought Audyssey "respects" the limitations of your speakers?
Audyssey only "respects" the acoustical response. If it finds output below the tuning frequency of your ported sub then it will apply boost or cut depending on what's required to achieve a smooth in-room response. It does that down to 10Hz.
In your case, where you deliberately boost low frequencies, I'm pretty sure MultEQ equalizes below the tuning of your speaker.