Recommendation Weight on Top of Speaker

one inch granite. should cost no more then $45 a square foot. I use such on top of ProAc's. Did improve the sound.

Not too argue.
The speaker pistons create vibration.
Vibration is not helpful for good sound.
Mass will absorb vibration.
A leather bag of lead shot and sand will work well.
As heavy as structures allow;in this case there's
no such thing as "too much of a good thing".

Weight can be beneficial when utilized properly. Exact amounts must be determined by experimentation. I happened to own some old VPI bricks which were and are one of the best solutions for speaker dampening. Mapleshade heavy hats are also excellent and look cool as well!

the speakers are fine with nothing on top. the sonus faber cabinets are superb and solid

Whatever you do, don't let your signif other put a plant on top, no matter how "good" it looks or sounds!!!

I don't know much about the Concertos. If your bookshelf speaker has relatively thin cabinet wall, you can cut two pieces of 1/2 inch to 3/4 inch plywood, about the same size as the side wall, place one piece on each side and clamp them down with a pair of C clamps. That will definitely clean up the mid bass. The draw back is that it doesn't look very good.

maybe these would work ?
http://www.mapleshaderecords.com/audioproducts/brassweights_hub.php