You would probably need to use some kind of bonding compound like woodglue, silicon, or something to seal up the seams of the speaker. You would need to remove the woofer to do this. Take care not to damage the gaskets and screw holes as air can leak from there as well. Just putting foam in the bottom alone won't fix the leaks, but it could improve the sound a little if properly applied. Those speakers don't have any cross supports inside the cabinet which makes for a very resonate cabinet (like an acoustic guitar).