Custom rack, isolation under platform vs component?

Looks great!
Check out Nobsound springs on Amazon for under the rack.  They’re like $35 delivered and four of them will handle up to 320 pounds!

Nicely done.  Woodworking is definitely a skill, even on something fairly straightforward as this, takes a lot of practice! 

I like the first iteration - looks and concept.  Assuming the mortise and tenon joints were just glued?  If all the joints were screwed into the vertical legs, I think you would have obtained the structural rigidity needed.  As well as using beefier dimensional lumber for those horizontal pieces.(1.5" x 2.5"). 

I'm all for isolation as well and don't let the naysayers deter you from your design.  For very little cost, you could use springs under those platforms.  You have the background to figure out total weight of the component and platform to get it down to single digit freq (3 - 8hz ~).  As long as compression is right, you're golden! 

Seeing as the platforms are now permanently fastened, you could still make use of springs under the components themselves (directly or via another smaller board), at least as a test.

Thank you. And I added several metal brackets and I am almost completely satisfied with the rigidity. I have another set in the mail, and I'm confident that after drilling those in, it will be good. It is a solid, heavy rack where it stands. And I have Alto Extremo and Stillpoints feet for the components. Thank you for the suggestions.