I too have found MDF to not fully hold screws or threaded inserts. But not all wood based products react so. Baltic birch plys,mahagony ply all hold screws and threaded inserts without backing out with time. MDF is truly a poor choice for a loudspeaker cabinets but its priced right very easy to CNC is gental on CNC bits.And you can paint it. This is hard to do with other wood based products used in loudspeaker cabinets. I only use MDF if customer requests paint and I let them know the painted cabinet will not sound as good as natural finish veneered baltic birch ply. And fasters used go through MDF baffle and are bolted. If threaded inserts are losening you can use lock tight or other thread locker in threads. I use this on all threaded inserts, fasteners still can be backed out with ease but wont losen with use.