This is not just another instrumental concert. 
Thinking, with deep respect, of all the venues around the world, and all the concerts, probably one where the musical act (of any kind) is performed unplugged in a lava cave on an active volcano is not something we all do every day. 
Think of the atmosphere, the history, the privacy, the feeling, the mood, the ambiance, the architecture, the age (almost three centuries), the exclusivity and, last but not the least, the vibration of such a place! Indeed something a bit special!
What about the acoustic! The hang is an acoustic instrument by definition so given the proper room it will sound divine. Also, the location is irregular so the sound spreads in an unmathematical way, adding more warmth to the music.
Speaking of the instrument, the hang is truly a peak reached in a novelle industrie that is also led by it. David Charrier is bringing to Santorini four of them because he is going to perform on all of them, sometimes simultaneously, since each is tuned differently.
Last but not at least - the ambiance, with candlelights and gorgeous arts on the walls, in a quite exclusive event with limited audience, and all of them are nice people (you just cannot love the Hang and be a bad person, right?).