cones or sorbothane or both?

In my humble opinion, the best, easiest, and not necessarily most expensive, is Zoethecus. No need to fuss with cones, blobs, etc. A really well engineered rack should be all you need, and I think Zoethecus fits the bill.

Contact Ken at Neuance. He does a shelf that does a fantastic job without a rack.

Hey, what about sorbothane layers between stainless stell washers (the interface between different materials should send a reflection of the vibrational wave back into the sorbothane for further dissipation). The place stainless steel ball-bearings at the top to couple your system to the washer-sorbothane sandwiched structure. I think ones can do this for about $8 for four sets with three layers of sorbothane. I am tring it this weekend.
Salsero

You can either transmit vibrations, or absorb them. Most soft or rubbery materials absorb the vibrations, and then re-release them back into the equipment. If you provide a direct coupling to allow the vibrations to exit, then you do not re-release them back into the equipment. I use Audiopoints, which are more than a simple cone, and are patented and engineered to do the job of evacuating the vibrations better than others I have tried. They look like simple cones, but the materials and geometry make the difference.

Correction to TWL. If a rubbery material absorbs the vibration, the energy is disipated in the form of atomic vibrations. Once the energy is absorbed, it will not be "re-released". This is based on the visco-elastic model of vibrations. Some energy will make through the rubbery material either if it is in a frequency where the material can't absorb it (i.e. the resonance frequency) or if the energy coming into the material is to high for complete dissipation.

Salsero