Here's a bit of the physics behind racks:
Vibrations come from 3 sources - outside (wind, street noise, earthquake), non-related inside objects (people talking/walking, appliances), or of course the sound system itself. Isolation is the obvious answer for outside and extraneous inside sources. So lets move onto vibrations from the sound system itself.
Often neglected are the extraneous vibrations created by the system. The vibrations caused by microphonics from the individual electronic parts and spinning discs make what goes on inside your component a well designed source of vibrations. Don't forget that the highest spls in your room are found inside (or in the case of an open baffle, on) the speaker cabinet (which is why some locate crossovers away from the speaker cabinet.
Vibrations are transmitted via the supporting elements, the air, and within the source components themselves. If you keep the sound pressure levels (spl) down, the airborne vibrations normally aren't a problem (air being a relatively poor conductor of vibrational energy). If you like it loud, the equipment should be somehow enclosed or isolated. Some folks have experimented with removing component covers, but in light of vibration control that makes little sense (perhaps the metal is influencing the magnetic field of the transformers). When you look inside any component you'll find many, many examples of large and small cantilevers (capacitors and resistors are typically connected to their boards like little springboards) and some examples (for instance CD/vinyl without a heavy puck or clamp or long runs of wiring) that are just laying there with no coupling.
A rack can only help reduce vibrations from supporting elements, but can help drain all three.
Regarding reduction in vibrations transmitted via the supporting elements - the rack is only one link in the chain that starts with the earth and includes foundations, load bearing members, and the floor. Which element is the weak link can only be determined on a case by case basis, but typically its the floor. Suspended wooden floors flex much more than any other common structural element. Unless you build something intentionally flexible, the rack won't be the weak link.
Drainage of vibrations involves - mass, anchorage, and stiffness.
Some prefer the rack to isolate rather than drain, this should be based primarily on the floor. A concrete sla rade makes for a wonderful sonic drain as it has plenty of mass, anchorage, and stiffness. A suspended wooden floor has mass and anchorage, but little stiffness. There is little point coupling to a bouncy floor.
As mass increases so does the amount of energy required to vibrate (move) it. Anchorage (or coupling) allows for transmission of the vibration into a combined mass. Stiffness is largely a factor of design (shape and bracing of solid objects). A diving board for instance could have the same mass as a cubic shape of the same material, and even though both are securely anchored its obvious which is more stiff.
Note that because of gravity, most after market sonic support elements treat horizontal and vertical vibrations/movements differently, but it reality vibrations are non-directional.
Finally we get to the real sticky part, frequency and resonance. Objects do have a natural resonant frequency based on material, mass, anchorage, and stiffness. Few materials used for rack shelving are truly homogeneous. Others have spoken of plywood voids, but even the same species of wood is not consistent. Plys imply
a composite sandwich of differing materials. Even stone products are made up of different materials. High density fiberboard may be the best commonly available material. Generally, as mass increases it has the secondary effect of reducing the resonant frequency of the solid object is reduced and as the resonant frequency goes down, the amount of energy required to resonant goes up. So anchorage typically helps, such as in the case of concrete floors or adding damping material to the sheet metal panels of the component cabinet. The goal here is to move the resonant frequency to around 6 Hz (an octave or two above the natural vibrations of the earth itself and an octave or two below the range of the sound system).
Bottom line:
1. Isolate yourself from the world as best you can, this will allow you to hear more at lower spls. Lower spls mean less airborne and self induced vibrations.
2. If you listen loud (BTW extended exposure to 85 dB or more will result in permanent hearing loss or possibly worse) consider enclosing your equipment or moving as many components as possible to another room.
3. For soft (wooden) floors use soft supports (pillows, air bladders) or consider using brackets that are secured to the walls (if the walls are stiff).
4. Use heavy/coupled supports on concrete slab floors.
5. Devices that behave differently vertically than horizontally (like flexi-racks or roller balls) make no sense and removing covers is counterproductive in terms of vibration control.
All that said, personally I keep the spls down and use a small $30 Ikea "Corras" nightstand for my simple system. My well isolated dedicated listening room has a concrete slab, so I left the rollers off the bottom of my "rack" so that the posts that the rollers attach to are allowed to act to some degree like spikes. I then use a 2nd matching nightstand as a side table at my listening chair that allows me to add a 4th shelf if needed in the future. In total a $90 and one hour investment. The "rack" elevates the controls for my convenience, permits short cable runs, and stays unobtrusive for imaging/soundstaging.
