Speakers on top of subwoofer

Is mounting a speaker on top of a sub a bad idea?

Yes.  Putting speakers on a vibrating subwoofer is always a bad idea and especially so when you’re talking about very expensive speakers like those Harbeths.  Subs can be very flexible in placement so I’d get creative with that to avoid having to use them as speaker stands.  

Thanks Soix.  You are confirming what my inner common sense is telling me.  

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