Well, it could be that the sub is particularly badly placed for that particular frequency. First just try moving it more into the room as an experiment, see if it not only stops shaking but also just sounds better. :)
subwoofer buzz - how to eliminate it
MartinLogan Dynamo 700w
At low frequencies, it vibrates, like it wants to fall apart. It does have a rubber feet, sitting on a hardwood floor, what could prevent it? Isolation feet? More rubber? Also it's facing the floor but can be turned sideways, would that help?