How important is a good rack?

The rack is a component. It should be of the same quality as the rest of your rig.

To answer your question, very important and maple seems to be the wood of choice. Maple dissipates vibration. Since I am in the midst of a complete system upgrade I knew my old wobbly Sanus rack had to go. I wanted a simple four shelf 48" wide solid maple rack but couldn't find anything without resorting to a custom built unit from a company like Timber Nation. Since a close friend is a professional wood worker I decided to build my own. As of this writing it is nearing the assembly stage. It will be solid as a rock and I anticipate it dramatically minimizing vibration over my current rack. The construction nearly eliminates fasteners since it utilizes a mortise and tenon design. I would be happy to provide feedback once it is finished. I would also recommend Timber Nation as they build custom maple racks. They advertise here quite frequently.

Well, frankly, since most racks amplify structural vibrations, even solid well constructed racks, the best sonic results will actually be obtained by placing components directly on spikes or cones on the floor.

Cerrot,
Your comment is simply stated and I believe, correct.

Keep the reports of careful ABs indicating dramatic differences coming! ;)

John