Being an evil person, our subs have been placed directly on a bare hardwood floor in what would be accurately characterized as a "live" room; lots of echos/reverberations. The subs are raised off the floor by an inch, raised on manufacture provided, little tapered legs.
My opinion, more than simple legs to raise the sub off the floor, the trick is to take the time to carefully and "ACCURATELY" integrate one's sub into a room's acoustics. And by accurate, that means, without emotional attachment. In my opinion, boomy subs is a sign of someone's subs being driven too hot for a room's acoustics and if this is the case, is responsible for much of the shake, rattle and roll that many here are asking about.
If one doesn't have a room analyzing program and their subwoofer system is guilty of being over driven or is poorly integrated into their room's acoustics, they'll never know the why of it all. Just saying.