Granite under wood for better isolation?

No, unless you have some visco-elastic feet or springs under your turntable, what you are actually doing is coupling (*not* isolating) your turntable to these materials. And every time you add another platform, you are introducing additional natural frequencies, since every material "rings" the loudest at a different frequency.

Schipo,
That is the most awesome audiophile story I've ever heard. The fact that I used to live in Manhattan makes it even better.

Schipo - Like Actusreus, I found your story fascinating and as they say, above and beyond the call of duty. You are a true audiophile in every sense of the word, not to mention, energetic beyond belief!!

Granite is Kryptonite to stereo systems. Use wood. Where your turntable is located is very important. Your turntable should be 2/3 of the way down the side wall from the speakers.

Schipo what you saw is not a spring at the bottom of the elevator shaft. In hi rise buildings we use whats called an oil buffer its a strainer using oil to cushion the cab when all else fails. In 2 and 3 story buildings you would see large coil springs in the pit. This should tell you that like in some designs oil is used with the build of certain turntable manufacturers. I have to give you the way to go award for your passion to get the best sounding vinyl front end. If you dont get up from that chair and try ideas you will never know what could have been better of worse for your system.