There are three reasons enclosures dance, The cabinet is too light, the cabinet walls are not stiff enough and lastly the cabinet is not firmly fixed to the floor. #1 add mass. Put a granite slab on top of the speaker. You can get cut offs cheep at your local granite installer. #2 This one is tough. You would have to thicken the enclosure walls. At that point you may want to consider another speaker. #3 Put three spikes on the bottom of each speaker and jamb them right into the floor. This assumes your floor is reasonably solid. Good Luck.
Mike
Mike