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 

This is not a problem. Simply use a vibration isolation pad/feet between the speakers and the subwoofer. 

Think for a minute how many expensive large floor standing speakers have large, vibrating bass drivers in them. Too many to count AND they have no way to eliminate bass energy going throughout the speaker and crossover since they are connected to the same cabinet as one unit.

One other big advantage is the subs would be positioned properly for stereo image especially if using higher crossover levels. ( Subs set with slightly higher crossover can be locally identified)

You also gain an advantage to isolate the two cabinets in your set up  and still properly aligned. Highly recommended.