Why do you want a riser? If it is to decouple the sub from the floor, I wouldn't go with the granite. In fact, I would be worried about resonances on any hard surface. I would put some kind of padding on the granite if you go that route, be it carpet, a rug, mat, something.
I just used the styrofoam blocks packed with my sub shipping box to bring it up a few inches. That was after I noticed the bass was stronger lying on the couch than at seated level. Does the trick nicely.
Then a "riser" won't fix that, you will still have sound waves. Hopefully. :R
Still, if you already have the materials, it doesn't hurt to give it a try. You don't like it, change it back. I don't think I would spend any extra money on it, though.
If you want to stop the pictures from vibrating on the walls, try putting some foam or rubber on the backs of the frames. I had issues with the light fixture in my bathroom vibrating, and I just stuck some thin foam pads between it and the wall.