What would give the best overall improvement

I have been considering new "feet" but there are so many options. I think a better outlay of cash at this time might be to stabilize my rack.

It's a VTI modular system, not really high ended at all. I have about 6 stacked units now with the TT on top. While I get seemingly zero vibration through the rack, at least nothing I can readily notice, there is a slight oscillation of the rack if you push at the top. I think I want to try bracing the rack to the wall.

The best bet would be to buy a multi level rack, but due to space limitations it needs to be "tower" like in its modularity.

Well, it seems there are no SDS controllers to be found. They are all out of stock!

Moonguy....I found that Terra Stones are not as good as my Bearpaws when used with my Superscout/rim drive. HOWEVER... what I DID find was that the Terra Stones in the front corners of my room did wonders with some room resonance I had. (discovered that by accident after ordering and trying the then useless Terra Stones)

For racks there is a simple test to see how much vibration is getting up through the rack into components mounted on the top shelf. Place a glass of wine or other dark liquid on the top shelf next to the component and see if ripples appear on the top surface of the wine in the glass. It might help, since the faint ripples can be hard to see, to use a flashlight in a dark room.

Guessing you already have a RCM or process in the mix...