If you have access to a marine supply company, you should take a look at some of the materials developed for sound deadening on commercial boats. For example, there is a very absorptive carpet pad that I installed in my car to reduce road noise -- this padding is used in luxury yachts, and it's very effective at reducing the transmission of low frequencies.
Another marine soundproofing material that I used in my home is a 2-inch thick blanket of fiberglass which has a 1/8" thick layer of vinyl in the middle of the blanket. (This material is often as a blanket on top of marine diesel engines, to reduce transmission of vibration energy.) I installed this material between the joists below the floor, and it did an excellent job of reducing the transmission of low frequencies between the living room and the areas below.
Last, if you have not palced some kind of isolation device under your speakers (i.e., cones, granite slab, etc.), you should do that. This will greatly decrease the level of acoustic energy that is being transferred to the floor.