@soix 

vibrations negatively affect sound, so adding significantly more vibrations into the speaker is just never a good idea, period.

Agreed, but that's NOT what I'm suggesting. I want to eliminate all vibration going to the speaker and it's absolutely possible. I agree that the tweeter height must be within range of your ear height. If it's too low the speaker can be adjusted higher using stands. The only problem would be if they are too high. You are assuming this mismatched condition is "Likely" when it hasn't been measured and we don't know.

@mijostyn   

Isolation feet are for the most part a joke. 

Believe it or not there is big difference in the dampening affect from different materials used in vibration control. It's not a joke, it's science.

Don't Set Your Subwoofer In The Corner see here!

Mike

@lewl28  Too low can easily be raised higher by the simple use of some speaker stands. I cut mine to an exact height.

What crossover do you set you subs? 

A thought to ponder... when I move subwoofers around a room I could always, easily detect where the sound was coming from. I could always point at their location. The higher the crossover the easier it was to hear their location. If used in a stereo application this can affect the stereo image negatively. By placing the Harbeths in straight vertical alignment with the woofers will retain the stereo image and depth. They will still be adjustable. Initial placement YOU control, change the phase, change crossover level, change the volume, toe them in or out.. Plenty of adjustments. For me the center image was more important than attempting to find the last ounce of bass and separating the subs and have two speakers off the center image. 

Many others feel this way also.

@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. 

I side with @soix on this. A speaker balanced on top of a sub is going to move. It may even fall off. This is avoided in 3-way systems by bolting, gluing, or otherwise fastening the sections together in the optimal configuration, which of course depends on the crossover. Which of these attachment methods should be used with an expensive speaker? I say, "None."

Then there’s the problem of stabilizing the sub on the floor, with an added weight on the top, raising the centre of gravity - it goes on. I wouldn’t touch it. YMMV.