Speakers on top of subwoofer

@gdaddy1 Yes, and it applied to items like automobiles. The job of loudspeakers is to create vibration. If the speaker enclosure is designed correctly it should not vibrate. Any vibration of an enclosure is audible and called distortion. Those feet are usually a hoax although there are some that actually work. When you change the height of a loudspeaker you change its relationship to the room and even a few inches can change the sound for better or worse. Only mechanical items like turntables and transports might be adversely affected by vibration. The best turntables have an isolating suspension.

@ditusa That is BS of the highest order. Subwoofers are up to 9 dB more efficient when placed in corners. The cone does not have to travel as far to create a given sound pressure level and that lowers distortion and conserves power. One subwoofer only in any location is a nightmare. The minimum is two. I place 4 subwoofers with a total of 8 drivers along the front wall ending in the corners creating a subwoofer line source. There are no room modes at all. Bass is even throughout the room except for a 2-3 dB increase a the boundaries. 

@gdaddy1 First of all, those Wilson's are incapable of projecting 20 Hz into a closet. Frequency response specs are extremely misleading being taken at one meter. In short, they are not subwoofers which for the majority are poorly designed, cheaply manufactured and vibrate like crazy producing vast amounts of distortion. 

Having said all that there is absolutely nothing wrong with placing a speaker on top of a subwoofer as long as nothing rattles. Isolation feet are for the most part a joke. I would put some self adhesive felt feet under the speaker so that both surfaces are protected from abrasion. Make sure the subwoofer is dead horizontal or the speaker might walk off with the vibration.   

If you want your speakers above the subwoofers build a stand that straddles the subwoofers