@lewm , It can work either way. I think the majority of people building subwoofers come up with a design concept, then choose an appropriate driver and finally given the parameters of the specific driver tweak the dimensions of the enclosure to suit.
If you look at the picture of my system on the floor you will see a woofer to the outside of each panel then in the center those two boxes on the floor are subwoofers. They are pointed at eachother. These woofers have 200 lb sealed enclosures with a Q of 7.6.
The new design is a decagon cylinder with 1.5 to 2" side walls (they vary). There will be a 12" driver mounted in each end. They will also be sealed. There is no reason to resort to ports when you have advanced "room control" which is really speaker control. With enough power you can make a subwoofer do just about anything. I would have liked to use larger drivers but space will not allow.
The tonality of a system is debatable and we all have our own preferences. What is not is image and detail. DSP allows you the adjust the frequency response of the channels individually so that they are exactly the same. This gives the best image and with it detail. Every audiophile that has heard it goes out and gets a processor and I am not kidding, every single one.
Lastly, there is no such thing as a "fast subwoofer" when a woofer is not fast enough it's high frequencies roll off. Usually, the cone breaks up first. Even 18" drivers can make it to 500 Hz. I think what people really mean by this is muddy vs well defined. Mud is coming primarily from the enclosure. Then there is the amp's ability to control the driver. Some amps are good at it, others not so hot. You need a powerful amp with a very low impedance output stage. Transmission lines are a way of effectively doubling the size of the driver at certain frequencies. They are very difficult to make and require a lot of trial and error tuning. It is much easier to use large drivers or a multitude of small ones.
