@lewm
I could absolutely guarantee to find you some Vaughan Williams you would like, just because he wrote such an immense variety! You's have to listen more than once, though.
The open-air plinths are usually based on slate or some such. Slates are a bit like graphite, very directional with vastly different properties depending on where they are quarried.
My existing SME plinth includes springs and foam damping, though nowhere near as springy and undamped as a Linn. If I raise my internal plinth, the springs and damping will disengage. Each layer will start with constrained layer damping between it and the next layer. When the springs are disengaged, I will rely on three IsoAccoustics OREA Bordeaux pucks to ameliorate vibration coming up through my Sydney sandstone blocks.
Sydney sandstone is quite soft and porous with about 4% cavity. There's plenty of it. It is up to 600 feet thick and started to be deposited before dinosaurs walked this planet. Enough has washed away to form the up-side down Blue Mountains and to drift thousands of kilometers north to form the world's biggest sand island, K'gari once known as Fraser Island, which is almost 100 miles long.
I am very open to suggestions, and have a gut feeling that the Garrard is valued because of its resonances!