I was bored, here's some food for thought....
Here are some responses:
12" Brahma mama in a ported(21hz) and sealed, gray and yellow, repectively.
Now, 3mH ine ach example:
2 things are effected on the ported enclosure and 1 on the sealed.
The ported enclosure has a GD peak around 50hz like the sealed one does, also the port resonance or near will have a higher GD aswell.
So now look at the FR>
Around 55hz they have their peaks in output with a SOON rolloff like a bandpass. It is not a perfect bandpass IMO and i dont even consider it one since the mecahnism is unconventional. The voice-coil just dosn't like to even play where the rolloff is because of it's high inductive impedance there so it is not an ACOUSTICAL rolloff in this area, but an electrical voicecoil one.
What is interesting with the ported application is that while in the middle of the response where Port and cone output combine constructively, you have less GD then either side alone(however this is true without Le, but Le does reduce GD further here)
Now, what about the GD increase near tunning? I am not sure, anyone?