I'm working on a sonosub with a shiva or av12 driver and the amp350 plate amp from Stryke. Basically I’m looking to make something very similar to a SVS 20-39. I have WAF to build it but no WAF to buy one. So I’m really looking for something with around the same performance. I’ve downloaded WinISD and the Adire LspCAD and have been playing around with volume/tuning etc. My question is how does SVS get away with their three 3” ports? I can see the advantages in greater equivalent port area to reduce air velocity in the ports (and the ability to alter the tune w/plugs). However it seems that the ports have to be really long according to the software. I have heard that a rule of thumb is for the port to be no longer than 5 x port diameter. So three-inch port equals max of 15-inch length. I can’t see how they achieve a 20 Hz tune with 15 inch ports. Is that rule really important (audible)? In addition looking at the basic spec.s of the 20-39 the internal volume should be around 90L (105L – port volume-amp cutout-driver) and the software predicts port length to be longer than the apparent internal dimensions of the enclosure (keeping the port at least 1 port diameter away from other objects) to achieve a 20Hz tune. Do they run the ports right up to the driver? Are they J shaped? Their design is very effective by all accounts and I’m just wondering what kind of trade offs they arrived at to make it work. It will make it easier to make the same sort of trade offs in my own design if I knew that I was in good company with SVS’s engineers. Any thoughts?