@elizabeth Aside from drilling and installing 1/4-20 T nuts, or threaded inserts, to accommodate threaded spikes - my first inclination would be to try a set of leather, shot bags. Make ea. bag long enough to go across the width of the base and maybe 4" or 5" wide and a couple of inches thick. Lay one across the front and one across the back of ea. speaker base.
I'm using some small (about 6" x 3" x 1.5") leather shot bags from my shop, laid across the metal feet of my Maggie 1.7s. Not only did it solve the problem of the speakers migrating on the carpet (only a quarter of an in. or so per mo. but enough to affect their off axis imaging) but also made an overall improvement in detail and air. I'm happy with the results, but now have to make some that are a bit more decorative and a little higher on the WAF scale...Jim
I'm using some small (about 6" x 3" x 1.5") leather shot bags from my shop, laid across the metal feet of my Maggie 1.7s. Not only did it solve the problem of the speakers migrating on the carpet (only a quarter of an in. or so per mo. but enough to affect their off axis imaging) but also made an overall improvement in detail and air. I'm happy with the results, but now have to make some that are a bit more decorative and a little higher on the WAF scale...Jim