Drywall screws will work also, but experiment with them on some scrap to get the correct pilot hole ( they can snap under to much torque.
In MDF, screws are used as clamps while the glue dries..they serve no other purpose.
You could remove the screws afterwards and lose no strength , the strength is in the glue joint.
You stated that you have already glued it ...if its holding ok adding screws or dowels would be pointless.
Of course you lose strength if only glued, because with screws, the masses are connected internally, so the surfaces wont peal off at a heavy use, because this is a powerful subwoofer, not for 8" driver but for 15". Its like gluing a washer in a surface, if you add a screw, then it's harder to remove it. Next time I am going to do 1 inch box joints, that's the strongest way to join mdf and theres no need for screws.
I thought a new method to join mdf, it uses lots of glue, what if you fill the pilot holes with glue instead of screwing them? once glue dries, it turns into rock, you may fill pilot hole several times until glue stabilizes and cover the area, however, I am still with the idea of using #10 screws as dowels.
Will try to make the test of the 6mm pilot holes filled with glue. Titebond dries as rock, I dont know if any other glues are the same hard. Have tried elmers as well, incredible woodglue as well. I may say that this is a better task for epoxy, but this one is expensive in big quantity, and I observed how woodglue has nothing to envy to epoxy, at least in wood usage.