You should only screw them down till they are fairly tight. If you keep going, you will just start chewing up the MDF and then you would need to drill all new screw holes. which would not be good. Thats what happened with my old JBL speakers. when i got them there were already a few screws that didnt have anything to grab because the stuff had already been ruined. although the material they used wasnt as dense and hard as the stuff axiom uses. Well, to make a long story short, i ended up just redrilling holes and mounting the drivers differently so that all the screws held on.
