Enclosure of KH120ii is very well controlled and not resonant at all - I would say less as the old model. When i have time i will do a few measurements with acceleration sensor.These resonances at low levels around 300-500Hz are usually cabinet/panel related.
They appear worse than they sound, because they are usually buried beneath the noise floor of the listening room. As SPL increases, the vibrational modes of the walls changes.
It takes Herculean efforts to eliminate all cabinet resonances, and really drives up the cost of manufacturing and end-cost for the consumer. When cabinets are REALLY bad, you get things like this-
But as my measurements don't show the THD peak from amirm I don't think it's a normal problem of this speaker.