I want to say, that it could be that your prefab box is not close enough to the specs of your sub. And that you can have alot of success, if you build box threw the specs of your sub.
And there a vew of subboxconstruction what you can build bandpass 4th order or 6th order and so on. Some of those constructions are known for better SQ, but for this you have to ask,
that idk what directly best/better box for SQ or SQL.
And its cheaper to build a box, than to buy a new sub. Think for a single 15 you have to pay 60 $ + your work.