Try listening to the shelf and the equipment with a stethoscope. Many shelves, especially MDF, pick up and amplify the 230Hz to 350Hz range, and depending on thickness, also "boom" around 60-100Hz.
The former is from the particles themselves talking- inside the material, and that is what cones avoid touching many of.
The latter is the entire middle of the shelf going up and down, and you either straddle that motion with cones or brace the shelf or lay that hunk of maple right on the MDF, so they damp each other for that trampoline motion.
There are other solutions of course, but those are the two modes of vibration in a shelf, assuming the frame is rigid. A perfect shelf would dissapte all sounds coming into it, and that is achieved, or approached, by looking at the various acoustic impedances of all the materials involved.
Acoustic impedance must decrease as you go deeper into the material- that is how one makes sound "go away"- via no reflections. Check out this link- the Java Applet in the middle of the page.
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/reflectiontransmission.htm
It shows how the % reflection changes with impedance mismatches- for ultrasonics, but completely applicable to audible sounds.
Best,
Roy
Green Mountain Audio
