You're not thinking this through properly. If the delay is short enough the ear/brain cannot distinguish it from the original sound. The closer the driver is to the floor, the shorter the delay. The higher up the driver, the longer the delay. You won't hear the delay as an echo, but it does effect frequency response. Typically a tall mounted driver will create a frequency response dip right in the middle of the lower midrange. Play around with this calculator to see how it works.