These speakers seem just odd.
The passive radiator is too small. Simple physics says that it needs to have an area significantly larger than the bass driver - its whole point is to take over shifting air at frequencies below where the bass driver has given up. I have a horrid feeling that the passive radiator doesn't even have a spider - the really cheap small ones don't. Which may be why the additional mass is apparent held on with a screw. A too small passive radiator is going to run into excursion problems very quickly, and is probably the source of a large fraction of the low frequency distortion. (I wonder, if the passive radiator is indeed spiderless, whether it is hitting a rocking mode near 500Hz, and that is where that distortion hump comes from. Just a thought.)
If this speaker is intended to be used with a sub-bass system, why does it have a passive radiator at all? (This is a question I have I of any small vented speaker intended for such use.) A fourth order alignment is intended to allow bass extension below where the excursion limit of the driver would prevent the speaker going. It has pros and cons. If a speaker is to be used with a sub, design it sealed. It will be cheaper, and you will be less likely run into unfortunate compromises. And this speaker sure has them. Despite the passive bass radiator it doesn't even seem to have enough bass extension to get into a region where it would match with a conventional sub.
One can only conclude that this is another example of a speaker designed by the marketing department.