I've been thinking about something similar myself after some suggestions by Martin. However, my floor slopes somewhat and I use spikes to level the racks. To do the same with the cones would be trickier. My 'solution' has been to isolate everything that is on the racks either using Townshend Platforms and Pods for source equipment and amplifiers and, more recently, Black Ravioli for PSUs (more is on order to complete this). The shelves have also been decoupled with industrial vibration damping material.

Good question. We bought it ages ago from a company called Vibration Control, but I think they were either bought or have gone under. We used it for some optical isolation benches to prevent medium and low frequency vibration affecting the very long exposure times used when imaging using high magnifications on a microscope. There was some left over in a drawer so I used that to try and damp any airborne resonance. A seismograph app. showed it to be cutting out direct transmission and it certainly stopped any 'ringing' on the sides of the new cabinet. As to whether it cuts out everything I cannot say. I think you'd need a proper micro-seismometer to find that out.

Mike, if you go for the cones that have threaded holes, you can put a threaded bar in that's as long as it needs to be to ensure you have enough thread on each corner to ensure that levelling can be done.
Old racks using Imperial threads? That's not really likely unless they are of US origin. Which brand/s of racks are we talking about? If you're in doubt, then surely it's easy enough to find out?

Having had a look at the Clearlight Audio website, and thinking similarly, I'm not sure that using the threaded stud to adjust for height would be wise. The cones are a polymeric material and they seem to rely on the contact between the base of the cone and the bottom of the leg to provide structural integrity within the cone (and damp transmission). If you only reply on the contact between the thread of the stud and the polymeric body it could fail mechanically, although that would depend on the load they were under I suppose (it might also compromise the damping). A better way would be to work out the height differences at leach leg and then use packing pieces the same diameter as the base of the cones to take up the gap. They are fairly cheap though so might order some and have a play for use under the record deck rack - the frame looks to be too narrow to accommodate them on the new rack. I'm pretty sure the threads on the 3-leg rack are M6, but I have a spare layer at work so I can check fairly easily.

Looking at the design of the legs on the rack I can see that the outer tube is closed by a steel disc welded in with the thread going through a central boss also welded to the disc. The welds are good, but there is not a single flat surface. So, I'd need to machine the spacer to interface between those parts and the cone, which is doable but a hassle. I have some aluminium bar a bit larger in diameter than the legs so I might give it a go. At least I can check the fit on the spare layer and then just machine the height of the spacers as required, or maybe even make them 2 component so give some flexibility.

Mana and Sound Org Rack
Linn Sara stands, TDL speakers
Being cautious as British and maybe still hanging onto Imperial quite a few years ago.
Can also be a different size to the RDCs of course

Perhaps the RDC thread can be ignored as long as I can adjust height in the rack or speaker stand....

Those brands will all be M8 or M6 Mike. I still have some small top plate spikes with M6 threads from Linn Sara stands that I bought to screw into some stainless steel footers I made for an AR turntable that I modified. Sound Org stuff was all M6 or M8 depending on age and Mana is M8, I think you will find.

Derrick after I asked him now makes a black ravioli big pad to accept spikes.

Other varieties of tinned luncheon meat are also available. There's even a Trump flavour.