floor vibrates like a speaker

I am close to finishing my listening room. The walls and ceilings are two layers of sheetrock hung on resilient channel. The problem is the floor. My room is over the garage. The sub floor is 1" t/g particle board. It resonates like a speaker into the garage below, and although it loses some db, it makes its way through the garage walls to the neighbors. I intend to insulate between the floor joists but I doubt if that alone will add much to the STC value. I have seen lots of recommendations for sound blocking, including using expensive vinyl pads. However, the least expensive and one of the more common mehtods recommended is adding a layer of sheetrock (or 2) over the subfloor and then a layer of plywood and then put your carpet pad and carpet. I wonder how soundboard in place of the sheetrock would change the dynamics? Your input is as always, welcome.
How about trying some Granite underneath the speaker footprint. Maybe like 1" thick ,that should help. Try www.oregondv.com He usually has some stuff kicking around that might help. I purchased some gorgeous Granite amp stands from him. He can also do any size you want..Good luck
In my living room I have a floor that is very dead. My intent was to block the sound of an ancient oil burner furnace located directly below. (It sounded like a Saturn 5 rocket when it started up!)

The floor consists of 1" boards with oak flooring over. (This was the original floor 150 years old).Then there is felt (not rubber) carpet underlayment, covered with 1/2 inch ply, nailed down through the felt. Then, in the center of the room, comes the regular carpet underlayment and the carpet, and around the carpet it new oak flooring.

The floor is, as I said, very dead, and I credit the felt carpet liner/plywood layer.
I have just moved into a new home with my stereo in the bonus room over the garage. The only mod (other than a dedicated breaker for the receptacles) I was able to do as we built the home "long distance" was to the floor. The av4home dealer in Columbia, SC recommended to add another layer of plywood to the 3/4" OSB flooring. this is done over roofing felt and then screwing the 1/2" OSB into the first layer NOT into the joists.
I am very happy with the results. This is the best sounding room I have ever had. The second recomendation was the double layer sheetrock, but I was not able to get this done.
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.
Oldears, I am not familiar with the accronym "OSB", is that a brand of particle board, plywood or something else?
There is a simple answer. Black Diamond Racing Pits (carbon fibre material $60 each). Will make speakers sound better anyway and with deal with the floor movement.