the way the port is tuned changes with volume and port area.
say if you had a port tuned to 30hz, with 18"^2 of port area, it would be louder than one with the same tuning and only 12"^2 of port area.
and lower tuned ports have better sound quality, while higher tuned ports have more output.
I think from my experiences and listening to other peoples stereos that about 32hz is the best trade off of spl and sq. but that is entirely opinionated.
I listen to more rap and stuff with lots of bass. If you listened to stuff that called for more response (rock, classic, jazz) anything that makes sq shine, you would be better off ported to about 27 or 28 or go sealed.