I've been using granite ( drilled holes @ 3/8 - 16" ) underneath my speaker stands since 1988, and continue to do so to this very day, yet I've gone a few steps further and added Track Audio Spikes under both the slabs of granite and stands, as it allows me to adjust the height on both, and atop the granite I've placed a set of Isoclean Power TT-008 Tip Toe Bases.
And then seeing a few of my friends do the same, in one case as ex-friend added slabs under his Harbeth Compact 7ES-3s and Skylan Stands............, and it snapped the bass notes together in a manner, I didn't know they were capable of, whereas before the slabs, to my ear it seems as if their bass notes where always 3 - 5 beats behind the treble and mid bass if you will.
I know some prefer some sort of wood ( Baltic Birch or Maple ) underneath their speaker/speaker stands, as some feel granite rings!, yet to my ears, I think not!, yet I tend to be from the mass loading school of thought as opposed to the rigid and light loading mindset, in which case, I'd say the truth lies somewhere in between the two - but granite has a weird way of snapping into focus both the sound staging and hidden nuances as presented at the speaker end in a manner that draws one deeper into the backdrop more readily - at least to my way of hearing/feeling musical notes.