port shape - circle vs. thin slot, why?

My velodyne subwoofers have the same slot port, same wih my old definitive technologys prosub 200. same thing.

was always curious about that myself

Hi there, I am also a velodyne owner and I do know the reason that they have a slot port. It is so that all of the air displacement is directed at the listener as opposed to tube ported designs that have the port at an opposing angle to the sub therefore directing the sound waves away from the listener. The tube ported designs are tuned to produce a small frequency band really well and the rest only mediocrely and the sloted port design tends to be more frequency friendly than the tube ported.
Both of the designs have their strengths and weak points such as in smaller listening enviroments a downward firing rear ported sub has more of an advantage because driver noise is greatly reduced as opposed to a forward firing woofer that is much more revealing in small listening enviroments. But this can be solved by facing the speaker and port side to a corner which works very well.

Slot loading tends to reduce port noise, which is derived from turbulence within the port. Depending on how the slots are achieved, it can also be less costly and time consuming for the manufacturer to achieve as compared to a tuned duct. The best ports have radiused inlets and outlets with "dimpled" ( like a golf ball ) surfaces inside of them. These are also the most costly ports to make and use, hence their lack of commonality on the market. Sean
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Miller and Kreisel has a new pair of active monitors with a plug you can put in the port hole (round) when using the speakers with a sub. What do you think of that idea, Sean?