I have a Auralex Acoustics Subwoofer Acoustic Isolation Platform. If your subwoofer is on a hard floor (wood, tile, luxury vinyl plank....) then these work great to improve sound and minimize vibrations. I loved mine when I had luxury vinyl flooring but now I moved to a house with carpet and no longer need it. It's in perfect condition.
Update: will throw in 4 acoustic sound panels. See last picture