For turntable, I went for "sitting on large and heavy but isolated with some sorbothane feet (
https://www.amazon.ca/gp/product/B00AJ2FVHG ) between turntable and large heavy thing". Seems to be working really well so far... historically I have ended up mounting turntables on wall-mounted shelves to get enough isolation from the floor.
For shelves, I don't have a lot of answers yet (I need to replace the old coffee table I'm currently using at some point) but I noted that tempered glass seems to resonate at between 1KHz and 3KHz depending on thickness. Didn't seem like an awfully peaky resonance but if you really wanted glass shelves I guess one option would be to get laminated glass, ideally two different thicknesses with an absorbent layer between them.
Predictably, I'm thinking about building shelves from 2x* pine
