This is certainly a very good read, there are just a few things I wonder about though.
I have always had the impression (using my ears) that sealed box subwoofers sound better and more natural than ported subwoofers, in general, it's hard to be 100% firm on this, because there a well designed and poor designs in both camps, but fur music, I would pick a sealed sub
I'm getting the impression from this article that driver size is unimportant, if you look at the physics, I don't seem to undertand that.... An 18" driver generally has like 3 to 5 times the mass of a 10" driver, and if you have to pair the sub with satellites with 4" bass/mid drivers, I don't think the 18" will provide for a good match
A ported subwoofer will have a much more significant phase aberration and group delay compared to a sealed box, because of the much sharper roll off..... Whether this is audible or not...... People might start a war on this subject...... I certainly know that it's a fact, but whether it's important or not should probably be up to each of us to sort out. If you can hear a difference, then it may be important.
Personally I believe this may make a difference.
Whether you go sealed or ported depends on the parameters of the driver you are working with. A sealed enclosure will have an F3 somewhat above Fs, but roll off will be 12db per octave. However with sealed to get good output at 20 Hz some Eq is going to be required, and that raises the specter of thermal compression.
The ported enclosure can have F3 very close to Fs. I agree there will be some phase and time delay problems below Fb, but I think these are not highly objectionable. You can tune a ported enclosure so the bass does not splash all over the place. The other problem with ported enclosures is that the driver rapidly decouples below F3 so you can easily bottom the driver without a subsonic filter. So it is swings and roundabouts.
I personally favor the TL for the most natural bass, however TLs with F3 down in the low 20 Hz range occupy significant real estate. The support is broad however, and roll of 12 db per octave. I have found that TLs with F3 in the neighborhood of 25 Hz really give you bass you can feel without advertising it, and touch lightly until there is serious program in that range.
Once people experience bass from a TL they seldom go back to anything else. Dual lines tuned 1/2 octave apart give nice smooth natural bass over 2 1/2 octaves. I'm not sure why, but they just don't boom at all. It is smooth deep even and effortless. So many of my friends notice that.
Nowhere in the formula for deriving F3 is the diameter of the driver relevant directly. However larger cones tend to be heavier, so you have a heavier weight on the spring so to speak. However you can make the spring less stiff and accomplish the same thing. The vintage JW driver, had a four inch tractrix aluminum cone that weighed only six grams. The suspension was unique in that the cone and voice coil were suspended under slung on three long beryllium cantilevers. This JW module was capable of quite astonishing bass reach.