I have a set of rather large and heavy bookshelfs,
JBL Studio 38IIs on top of glass shelves, but hidden between the shelf and cabinet, there's a double layer of toolbox rubber/foam lining (sold at auto part stores). It is sufficient for my needs, but I live on a 3rd floor, so I don't really ever "open them up", but I have played music quite loudly, just not "alone in a bungalow kind of loud"... I never went beyond -28 on HK3490 . It doesn't show, and I never had any noticeable vibration issues.
Link to picture
Your mileage may vary. They are about 36 inches of the floor, and are my main "music" speakers.