First one can be a somewhat waste of time, but different perspectives never hurt. Just gotta make sure the logic isn't circle logic or based on something untrue.

Second one, we have one in these boards here.

Third one:

Tuning of a ported subwoofer box is determined by a combination of port area, port length, and net volume of the subwoofer box.
Completely ignores that the sub's T/S specs determine how effective the box is and if the port is even useful for the sub. (in otherwords how it reacts to power at frequency in a box at varying sound levels)

Personally if 30-40 Hz is three times louder than 20-30 Hz or 40-100 Hz then it sounds like crap. Peaky subs do not make for good sound quality. They do however make for great SPL subs. The most impressive systems are the ones that bump equally at all frequencies yet so clean you can hardly notice its loud minus the fact it affects your breathing, shakes your clothes and the car is being shaken apart.

Fourth link, thats a good one!