To determine whether you put the single cone in front or in back, take two cones and place one at the midpoint of each side and see whether the component wants to tip forward or backward. Place two cones in the direction of tip (usually the back but not always.) Using three is usually better than four to prevent wobble. Note (exception): if you're working with a CD player/transport, get one cone under the transport mechanism, and the other two wherever they will provide the best three-point stability.
All that said, cones (also ball bearings, or rollers) under components (not the same as under speakers) act as mechanical "diodes" to "siphon off" real mechanical vibration in one direction but not let it come back the other way. This kind of vibration most often occurs in things with motors like CD transports/players, cassette machines, and turntables. Electronic "microvibrations" like from tube microphonics and transformer hum are more easily gotten rid of by absorbing them and converting them to heat. This is best accomplished with some kind of elastomeric compound as found in Sorbothane gel (Audioquest pucks or PandaPaws) or rubber, like in Vibrapods.
The third kind of vibration that all types of components are subject to is environmental (basically floor-borne or air-borne.) The best way to deal with those is to address the surface (shelf, rack, or whatever) the equipment is sitting on, and make sure it isolated from, or very solid and resistant to, these environmental factors. Putting stuff under the equipment itself will not help if the surface it sits on vibrates.