What makes a good equipment rack?

I would suggest solid construction, damping, and flexibility for different sized equipment.

I had good success with a Sound Anchor equipment stand, which is solid, welded steel with a sand filling for damping.  I currently use Sound Anchor stands beneath my monoblock amplifiers and beneath my stand-mounted speakers.  For reasons of aesthetics, I now use a purpose-built wooden furniture cabinet for my front end gear, but I have significantly reinforced the cabinet making it very heavy and solid.  Wood seems to have some inherent damping properties but I also use a variety of damping platforms on each of the shelves, including one carbon fiber Black Diamond Racing shelf and three Z-slabs, which are constrained layer platforms that originally came with a Zoethecus equipment stand.

I like the ability to adjust the shelf height for use with different equipment. My wood cabinet originally had adjustable shelves but for stability a have since mechanically anchored the shelves to the frame of the cabinet. However, I can adjust them to different heights if I need to in the future.

If you start with a solidly built stand (or rack), you can then experiment with various equipment footers (such as Stillpoints, etc.), platforms underneath the gear, and damping on top of the gear.  You can also experiment with different footers for coupling or decoupling the stand itself.  

Speaking of racks and stands, and since you are good with underwater stuff, do you know much about the SRA folks and products?

@geoffkait 

didn’t those Zoethecus platforms originally use Sorbothane for the viscoelastic material

Search me Wally, I have never taken one apart.  I believe they are constrained layers and I know the top layer is aluminum.  They are of course no longer made but people seemed to like them.  I no longer have the rack but I have three of the z-slabs.  I found the following quote at the link below.  Who knows what they mean by "phenolic, viscous damping fluid" ....is that code for Sorbothane?  Now you have me interested.  Maybe I will go take one apart.  

The heart of the Zoethecus system is the Z Slab. The hand-laid composite core consists of 10 discrete sub-layers of phenolic, viscous damping fluid, aluminum and high density fiberboard to thoroughly dissipate component resonance. This top-of-the-line 15-pound slab is perfect for heavy amps, transports and turntables.

https://store.acousticsounds.com/d/13194/Zoethecus-Z_Slab-Equipment_Rack

When I was using my Sound Anchor equipment stand, I made a quartersawn oak top shelf that really warmed up the industrial appearance.  I also made thick quartersawn oak platforms that I use on the tops of my Sound Anchor amplifier stands.  I use thin sheets of Isodamp elastomer material between the wood platforms and the tops of the amplifier stands.  The stands themselves are spiked to the concrete floor.  You can see the various stands in my system pictures.