Down-firing Sub makes floor rattle

I just bought a ml dynamo sub. it is a great sub but it makes the floor rattle. i only use it for music so it is annoying..how do i fix this? i don't have it on the attached spikes as of yet....it is on the attached stand with rubber feet. would the spikes solve this? or should i just put something under the sub? this sub can also front fire, but i'd like to use it down-firing.......the carpet is very thin and maybe this is the problem?

thanks