Speakers on top of subwoofer

Another thought is that subwoofers and speakers are different sizes, so placing your speakers on top of your subwoofers might make them too high or too low for proper listening.

FWIW, I would never put my speakers on my Sub.😀

It's not optimal, but it's far from the worst thing you could do.  A bowl of water can help give you a visual impression of how much vibration is present on the top surface of your sub.  It may not be a lot.  You'll have plenty of options on how to decouple the sub from the Harbeth.  I would start with rubber/cork HVAC pads.  They are a few dollars at most hardware stores, or you could go audiophile approved expensive.

Mijostyn made a good suggestion.

@gdaddy1 

Good counter example.

Then, according to this statement, you can conclude that the Wilsons can "never" be correct. Since the speakers are always connected to the subs and according to you, they shouldn’t be.

@gdaddy1 No, and as with most things in audio it’s a compromise. While the designer gets to implement the crossover and there are some advantages of things like potentially better impulse response, it comes at the high price of having to design/build much heavier/more expensive cabinets and placing bass where it’s not optimal and most likely creates significant bass problems in a room. And in this particular case, which is what we’re really talking about here, where the OP already has subs it would be both silly and counterproductive to place them where they most likely will not sound best and cause room-induced bass issues. Plus, putting a speaker on a big, vibrating beast if not necessary is just a stupid idea — vibrations negatively affect sound, so adding significantly more vibrations into the speaker is just never a good idea, period. And on top of that and as someone else mentioned, it’s also likely the speakers will not be at the proper height plopped on top of a subwoofer, so for all these reasons it’s just an awful idea to put those excellent and expensive speakers on subwoofers.

@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.