Slightly above ear level seems ideal to me, and usually happens without me trying much. I can’t think of a time where the sound seemed to be coming from lower than my ears, even when the speakers were. I have some binaural recordings which are meant for headphones, but the height effects work incredibly well on regular speakers. Recordings of airplanes flying over seem to be way up above me. Birds up in the trees too. For regular music I’ve got little use for that effect, but it proves to me that height channels are not necessarily required for vertical special effects.
