If the floor seems to be the offending item, I always recommend fixing the problem, instead of adding some other item into the mix to try to mitigate the floor problem.
Nsgarch has some good advice about locating the speakers over the joists, and there can be supports placed under the joists too.
Nsgarch has some good advice about locating the speakers over the joists, and there can be supports placed under the joists too.