Custom rack, isolation under platform vs component?

Either should be fine.  I like the springs under the platforms so long as they are stiff enough to handle the weight.  The higher mass will be excited less to vibrate and the springs should do a better job of isolating.

The exception would be if you listen at really high sound pressure levels so the platorms could be vibrated by sound waves. 

Jerry

Very nice looking rack. I built mine from 3/4" black pipe and butcher block and it is very sturdy. Thanks 

Very nice. Add some angle bracing on the back and sides, or a small panel and it will firm up nicely. 

Thank you guys for the advice and compliments. I bolted the platforms down and used some steel bracing for support. It is now a pretty heavy and sturdy rack, and I pained the frame black to match my speakers and components. 

Looks good