My last comment is that there are a great many locations around the room, not just behind the speakers, where acoustic resonators are effective. The opposite wall, side walls and upper corners are the best locations. However, I kind of hate to say this, the *only reliable method* for determining locations for resonators like tiny bowls or bells is to use a test signal or two plus SPL meter to map out the entire 3D space of the room, finding points on the walls or wherever when the sound pressure is 6 dB or higher than the average sound pressure level in the room. All other methods will only get you part of the way or even make the sound worse than before. Especially when you put up a bunch at one time. Even the listen a little, move a little technique cannot compete with the SPL meter and will only find what we call a local maximum, at best.