Hey hilde45...
In my long experience as an Anti-Resonance Cop (especially under the hood) masonry rarely works. If your floor carpet is thin enough, you might try to find some vintage golf cleat spikes and weigh down the speakers:
50_31067b_lg.jpeg (1200×1600) (iconicauctions.com)
I've also been pleasantly surprised with bookshelf monitors atop carefully weight measured Vibrapods, isolating in a somewhat 360 degree fashion, despite most preferring mass loading with loudspeakers. Aluminum isolation cones can work well and are reasonably priced. As stated, adding some mass to the top of your loudspeakers can prove effective depending upon the approach used. Sound dampening the boxes internally and/or externally can prove extremely rewarding.
More Peace, Pin (bold print for aging eyes)