Raising them off the floor will cause cancellation and you can calculate the frequency at which that cancellation occurs by taking the difference of the hypotenuses of the driver and its imaginary reflection buried in your floor. The reflection is buried in the floor halfway between you and the speaker at a depth equal to the height of the driver. Divide half the speed of sound by that difference and that will be the null frequency.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated