This Tonewinner outfit have ne credibility with me. They issue totally meaningless specifications.
Just look at this picture of a dual 15" sub.
I would bet that sub is grossly overported. I have a strong suspicion that design has zero engineering validity. Those ports look like they were sized and placed by the cosmetic team and not engineers.
That picture just screams:- "Got up to sell!"
BK of Southend, do quote either a -3db or -6db point. They are building for the UK market where rooms are smaller, so the market is geared to smaller form factors. There designs look appropriate not laughable like that Townwinner.
Because of onerous VAT and other taxes, UK builders have to build for a lower price point and will not compete with US subs.