I'm of a different opinion:
The sound your downstairs neighbors here is in the lower frequency because low frequencies travel through the air farther than higher frequencies.
Using MDF boxes will only amplify the lower frequencies as the box creates a resonating chamber. (how a Subwoofer box works).
There are shorter stands you can purchase from most hi-fi shops which should mitigate the transfer of low frequencies through the floors.
Of course, if you have a bass dial on your stereo, you could also reduce the level of low frequency by simply turning the bass down. Unfortunately, this is too much of a sacrifice, in my view.
Better to look for short metal stands with spikes that you can fill with sand. The sand and spikes will increase the amount of sound traveling through the room and reduce the amount of sounds lost to the floor.