In my limited experience
Digital stuff in particular benefits from isolation. I use an old target rack (fully welded, very sturdy, limited waff) with black granite shelves. I could hear the difference as soon as I put a low-mid market cd player on, more clarity, more speed, deeper bass. How does it work ? Dunno, dont want to know.
Conversely I hear no difference on where the vintage valve amps go, be it floor, coffee table, rack with chipboard or stone shelf.
I found a heavy weight / slab on top of the rack damped the whole rack, which doesnt do any harm. I get no problems with turntable on the top of this pile, which I now cant lift.
Interestingly (assuming those reading this are interested), glass shelving seems to improve speed and detail at the loss of bottom end, marble is better than glass, but it has to be fine-grained black granite for the full effect. Any rack etc will do.
I got my shelves made from offcuts from a stoneyard that makes a lot of fireplaces, one at a time as they made them, tried one first and was so pleased I went back for more.
I got the idea from a friend whod spent a years wages on a new high-end stereo, the effect of granite shelves with his was even more pronounced than mine.
With speakers, put a weight on top to damp them down, blu-tack them to the stands or floor if you can. It wont hurt, wont cost anything and may surprise wit the results.
Whats the betting that the next wammer thinks Ive written a load of old tosh ?