C’mon Juju, just because someone is new to the diy side of things and has some very common misconceptions, it doesn’t call for being condescending and generally dickish.
OP, Juju has given you a very good suggestion though. That 21” driver isn’t particularly designed for a sealed box, but will still work decently well. Running it sealed will allow it to integrate very nicely with your existing sealed sub. Yes, the driver is designed to be optimized for performance at higher frequencies, but it works very well down low too due to its large cone-displacement capability and high motor force.
Data-bass has actually tested that driver in a sealed box as well.
It will require significant eq to get the frequency response you want, and more to correct for room issues. The amp juju recommended does have full eq capability built in.
If you would prefer to split the difference, and gain significant performance with a good diy-style driver, but with the build and EQ already done, check out offerings from PSA or JTR.
Here’s a sealed 18” from PSA:
That uses the B&C 18ds115 driver, and will absolutely blow away the ML sub you linked at a similar price point. Yes, the sound quality will be as good or better too.