How to isolated stand from springy floors?

I currently have a relatively heaving stand with a couple tube amps and a turntable on it. My big issue isn't really sound quality but my concern over tube life. My Manely Stringray II makes an audible tube rattling sound when ever my fairly light self walks by and I hoping I can come up with a fairly simple and hopefully cheap solution to this.

What would you guys suggest? Damping the underside of the shelf the amp is on comes to mind or possibly some sort of iso feet for the whole rack to cut down on vibration.