If it's not a SQ sub it's an SPL sub. It's either fish or fowl, it can't be both. Well, with the exception of stuff like Volfenhag and Visonik which aren't either, they're just garbage.
Look at it like this, if a sub cannot produce usable bass below 30hz it's not a SQ sub. If a sub is made for competition (which the DD subs are) and can't compete in SQ because they won't play below 30hz then what are they classed as? SPL or bassrace which is essentially an offshoot of SPL competing.
I don't have to hear a duck to know that it quacks.