Brass footers for speakers

The idea is to brace and dampen the cabinet to yield tighter bass. If carpet piercing the vibration is "drained" into the floor.
The best part is that they look really cool even if they don't change your sound. For that reason make sure you get them large enough to be seen from the listening position. I use them on several of my components and recomend Edensound he is very good and very fairly priced for the larger footers. I am not kidding about that, he makes really nice large footers.

What effect does a concrete floor underneath the carpet have on the damping as opposed to a wooden floor?

Your footsteps would probably not make a concrete slab vibrate but probably would make a wooden floor supported by wooden joists vibrate.