Building a custom isolation equipment rack

<EDIT> Tomic was a step ahead.</EDIT>

I would suggest that you look a boat panels using fibreglass and end grain balsa. You can lay it up yourself, or just buy it... and then laminate the veneer and damping layers on top/bottom.
 

https://atlcomposites.com.au/category/30/Coosa-

I recommend Jordan Peterson's advice: anything worth doing is worth doing bad. Most people over-think things trying to make something perfect or at least as good as possible the first time. If you take this approach I would keep on the current track of at least making it look good. Sure 90% of the mesquite will be covered by components, and the rest will be barely visible in a dark room, but at least it will look good in photographs. Audiophiles talk a good game about sound but what they really want is showy photogenic glam.

The part about doing it bad comes in with testing. Like those crap speaker spring platforms I made. Hideous in the extreme but building them taught me what actually works. You don't have to build a whole rack to do this. The same vibration control sound quality you will get from a whole rack, 95% of it will be there on a single shelf, 90% on a test piece. You can make square or round test pieces of different materials and test them a whole lot faster, easier and cheaper than a full shelf of the same construction. 

I imagine it is a foregone conclusion you will be using springs between each shelf and the rack, and Pods or springs under each component. This being the case the details of the shelf and rack itself drop way down in importance leaving you free to just build with sexy mesquite. 

Make the legs out of ABS, coat with sexy epooxy polymerhttps://www.bing.com/aclick?ld=e8W6lhOFUq-tW49GxlFXCeKDVUCUxoD67fCQeZNJWG4-GyZZ_JHEb2J8gafi8trIamGXiN8maeGfoY6wYm2SB6nlvZfKVSi5Q41WTDCTLR6wvz8ovvditjbCvjjXHda4b3Z6HCrI3wTwUiVesngM9ue70AUX-NgfKi_UfzL1oPn9EWYH43PHeH28lM__NNiP5BhxY660rSr6dv-MORIs6jsAmMzW2iXWnH4HQY0NXkkYP4CGZw2Ogv6kK4TlxqCAIgmeamZUQue8gGTkaKRDoXfY0DG3257dBKa1k_MEaeUwEsG_gGpCV8J9GtiT7lRlThHyrXz0ISKl_bfORTwPuvKW-nSWRl2v_kjql_2BfXtAA6dRzksua8Oer5OTShotsDtYJ3wVSP_uxgGPhxAQWnASfl6tSXTS3pAT5Z3_Ts98DvyTYAO5DxkH4r35s_DP3RwnjRDNd9xePW83zE6XKNeNVhFcwJaaqDWXo3ntZnoij7jcuuRntybXu3Eft_a8WSXXTOBwNexhQYaQ-9HbLThfgxxQslAfuiUG-M2fxeGqsMVVu7YDe_ir8qwILWiyqTecydnKfThZnJ3UUHj-y7SIbS59QAY2UyateXHOHugsQP3fUQM_dNaN8Bp8nWcaS_GvQe3gnhOuPXmVLLd2Y4HaLo2LNkyA0cQEtFsT5HQyocSaKfrDSW1h5CRtuAd2A2krjvPISnbCJfbLruZ9xoeV7vYSiIwhUKP5A3MJaMqSaEaxOj3-XevVmtSiPLg-UXOh690A&amp;u=aHR0cHMlM2ElMmYlMmZzdG9uZWNvYXRjb3VudGVydG9wcy5jb20lM2Ztc2Nsa2lkJTNkZGU1MjVkOTFjM2UzMTExNTY4ZTUzYzAwMjg1Y2M4ZDklMjZ1dG1fc291cmNlJTNkYmluZyUyNnV0bV9tZWRpdW0lM2RjcGMlMjZ1dG1fY2FtcGFpZ24lM2RTZWFyY2glMjUyMC0lMjUyMFJNJTI1MjAtJTI1MjBFcG94eSUyNTIwR2VuZXJhbCUyNnV0bV90ZXJtJTNkZXBveHklMjUyMGNvdW50ZXJ0b3AlMjUyMGNvYXRpbmclMjZ1dG1fY29udGVudCUzZEVwb3h5JTI1MjAtJTI1MjBHZW5lcmFs&amp;rlid=de525d91c3e3111568e53c00285cc8d9, fill with sand, tell everyone it's some exotic unobtainium composite ten times better than carbon fiber and a hundred times better than (Google some impressive jargon, insert here) and no one will be the wiser.

First thing you need to decide if you want to drain or isolate. Basically you need to deal with airborne, vibrations from the floor and vibrations emanating from the equipment itself. Springs might help with floor vibrations which are probably the least significant but wont help drain any energy. Rubber and similar may suffer from some of the same issues. My thought would be to drain the energy as effectively as possible. Likely you will appreciate different treatments on different components. I doubt it will be a one size fits all scenario, but wood would not be my first choice for a shelf medium. Better than nothing is not necessarily a truly good solution. 

Thank you all for your input and suggestions.

I have springs already, and the wood, and a frame that can be modified made of steel extrusions with connectors etc.

Yes I plan on isolating between the frame and the shelves, as it's a known efective solution.

The wood is a facade, covering up what will be the mechanical isolation using engineered springs. Steel plate has been used, laminated to MDF for the shelves in the past. I am looking to make the spring placement adjustable around the center of gravity. I'm trying to stiffen the wooden shelves, to reduce bowing or sag, and get it to within close mass to the shelves the springs were engineered for.

My thought would be to drain the energy as effectively as possible. Likely you will appreciate different treatments on different components. I doubt it will be a one size fits all scenario, but wood would not be my first choice for a shelf medium. Better than nothing is not necessarily a truly good solution. 

A set of knuckles does a decent subjective job of determining resonance. And panels with a layer of green glue between then generally has some reduction of vibration happening.

I'm trying to stiffen the wooden shelves, to reduce bowing or sag, and get it to within close mass to the shelves the springs were engineered for.

Those engineered marine panels are pretty light.