The sonic resonance from the bookshelf placement is derived from the fact that the BL 4000 speaker sound is now contained, concentrated, and directed or focused out. Amplified inside a box, if you will.
One of the original B&O advertisements had a gold pair placed this way, in a relatvely small room. My concern, despite the Alps, was always heat build-up, but I understand they are constructed to dissapate heat very effectively.
One pair of BL 4000 next to our flatscreen were removed from the speaker stands. They now sit upright on a thick glass stand, on either side of a new Beosound 4, and sound better to us this way. The speakers are at head level for anyone in the living room, rather than pointed up at 15 degrees as they were before.
We also tried BL 4000 on stands, and from ceiling mounts. Did not care for either, but in another setting with different construction materials, who knows.
On glass, on granite, attached to a wall, on the floor, or spread apart, the speakers all sound different. The choice of course, is what sounds best to you.