I agree with Planet in that the design completely determines what materials you would choose. The posts that suggest room treatment to be of more concern than the rack are in my opinion wrong. If your hifi is not very good then that might be the case but if it is then the rack is as important as the source itself.
I would recommend coupling the rack and preamplifier, poweramplifier and speakers to solid ground with several different materials that are all fast in transmission, bypass components feet and couple directly to chassis prefferably taking into consideration where the transformers are.
The source should be isolated from the rack and therefore the other components. This approach maintains the vivacity and attack of music while allowing all the detail to remain uncluttered and without any harshness or 'dark' colour being added to the mood.
In all honesty, study the clever designs of specialist hifi racks and try to understand why they work and apply it to you design. Simple construction and large mass is not really an intelligent approach to vibration, the key is to remove internal vibrations from components whilst allowing the system to interact freely outside of components if that makes sense (in other words to create a place where the vibrations from inside the components and the vibrations returning from the speakers can 'mingle' outside of the components, a sink or pool if you like) The rack is one of the most important parts of a stereo because it affects every aspect of the sound of every part of the hifi.
I would actually recommend buying a proper rack after spending time listening in your home on trial because there is a lot of poorly designed stuff to be bought. If you buy or build the wrong rack you can ruin your sound and no amount of box swapping or room treatments will fix it. This is just my opinion from experience, I don't mean to insult or criticise anyone of you fine chaps.
