Need advice concerning audio cabinet shelves

What would make sense is to follow quality advice instead of trying to reinvent the wheel.

Sorbothane is real easy to over-do. If you are determined to do this then cut into circles or squares a couple inches across and space them out at least a foot or two apart so they create a small space. This leaves the glass and wood free to vibrate on their own, not together which is what you get with a sandwich lamination type approach.

If you like wood then you like wood, and you seem to be stuck on it. Which I can understand. I went through a whole long thing of guys saying how great this wood or that wood, or why the other wood isn't all that. Even went to the extent of buying great slabs of pine and oak and maple and whatnot and listening to the damn things. Anyone here came even close to my obsessive experience I sure haven't seen it, and I been looking a couple years now.

What I can say for sure, it is not that there aren't differences between the different wood species. There are. But those differences are swamped, utterly and completely overridden, by things like shape, dimensions, and construction methods. The vast majority of difference between woods is when they are whole planks, which they never are. The minute they are laminated this changes everything. Then the thickness, width, length and shape changes everything even more.

Think of a xylophone. Every single one of the "keys" is the same, only the shapes are different. What is "constrained layer damping"? Nothing but a fancy word for "laminated". 

This is a tiny glimpse into the knowledge base on which Nobsound springs and Townshend Pods are recommended. If instead of taking advantage of this you want to experiment with the same old same old: Oh well. 

The approach you're using- rubber, wood, cones- to ameliorate being on glass is a pretty good one. Glass adds glare and hardness, which you sort of cover up with the rubber and wood, and then try and get some of the speed and detail back with the cones. That's not bad.  

Your idea of adding a bigger wood shelf, or entirely replacing the glass with wood, will probably be a little better. No idea what you are looking at cost-wise for that. But pretty sure Nobsound springs will be a lot less money and absolutely certain they will be a much better improvement. Read the threads, these things are absolutely awesome for the money.

But honestly, all these options are baby steps. If you are looking to do something that will be a significant system upgrade, look into Townshend Pods. I would start with a set under the tube amp and take it from there. Not cheap but these are great strides we're talking not baby steps.