it implies a few issues if the subs vibrate - it must sound "shaky". I wouldn't solve it by fixing whatever it is connecting it to the floor, I would eliminate the vibration.
Vibration dampening
After reading the article on Herbies isolation products, I got to thinking: My sub has more vibration going on than anything else, at higher volume you could put your hand on top and it feels like a massager..I am using spikes on thin carpet with concrete underneath. Has anyone had positive results with these or other isolators? It is a Velodyne DD12.