One way to reduce the 'echo slap' from the downward force against the floor, is to put a cone on the floor under the woofer outlet. I have a parabolic cone (from an old cheap speaker I found at a Goodwill) that I filled with dampening material, the parabolic cone is about 10 inches in diameter.
It did make a difference. (it is like a funnel shape except the sides curve)
Making one out of wood would work, even if the shape was not perfect.
I'm not sure what difference you thought it made, but an 80hz wave is 14 feet long ... a 10" cone is completely acoustically transparent to a 14' long wave. Maybe raising the sub enough to fit the cone under it is responsible for the 'difference', not the cone itself.
As Nils mentions, the rigidity of the floor is a consideration. Some sort of damping of mechanical vibrations can help from exciting resonances in the floor itself. But large subwoofers can generate on the order of 2-5 hp, yes horsepower ... a 10 pound platform with rubber feet isn't going to do much to mitigate that.
Point being, don't worry about it being a tile floor. Just focus on placement and EQ if possible for the best integration.
