Recommendation Weight on Top of Speaker

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

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