I looked at each of the websites you reference and they all say that non-parallel walls will not support standing waves. I think this is great marketing lingo, I don't believe that standing waves in an enclosure can be eliminated. There will always be resonances and mode shapes in any volume. Maybe the non-paralel walled enclosures will have "weaker" modes as seen from the back of the driver cone compared to the nice rectangular enclosures, I don't really know.

The location of the driver and port on the front baffle can also be used to mitigate the number of standing waves excited. Stuffing also does a great job of attenuation. Having built one tall thin trapazoidal enclosure, I am not looking to tring and odd shape again. Nice and square is much easier.

Martin