Interesting that nobody mentioned springs underneath their speakers. I put 6 under my 70-something pound speakers, and the improvement was stunning, regardless of their cheap $26 per 4 of them. The bass tightened up noticeably, and the soundstage depth increased, and the imaging became better defined. I consider these a poor man’s version of Townsend isolation bars. I got mine from China, via e b a y. They are known as 4Pcs Turntable Audio Isolation Feet Pad Spring Sand HiFi Speaker Floor Protector.
They are very reasonably stable with speakers, but I will caution their use if you have kids and large animals in the house.