I still say a pair of Blau 12's would match that amp you currently have nicely. Its on you to build a box makes proper use of them tho. That box of rocks you got wont work too well.
That Amp you have is 600w into 2 ohm correct?
A set of those subs I mentioned in something designed right will be a big upgrade from those old subs imo.
Its a budget solution to your current dilemma and should give you the best bang for your buck.
Don't let the pricetag scare you away. Those $40 subs do ok and on your power too.
Just don't expect to make flex videos for YT like some do. That will go nice and low and sound good while doing it.