Demoed the acoustic revive schumann resonator at the Munich hifi show. They had two in the room, and kept switching them on and off. Every time they switched them off the soundstage collapsed and became flatter, the plucking of the guitar also didn't sound as good.
Also did a similar thing with the stein harmonizer, similar effect, but he had 6 in the room which he took out, and they are way more expensive. So the resonator is my preference.