I think it is more important to decide where to put than how many. If the purpose of using cones is to "drain" vibration than I would like to drain them as soon as possible. That means they should go directly under the transformer and CD drive because these parts vibrate most.
A more scientific way is to use a stethoscope. Just put a scope against the bottom of the equipment at various points and listen for noise. Place a cone directly under the noisiest point and place the others evenly to balance out the weight.
Spikes under speaker are a different story. It is much easier to make fine placement adjustments to a speaker with three spikes than with four.
A more scientific way is to use a stethoscope. Just put a scope against the bottom of the equipment at various points and listen for noise. Place a cone directly under the noisiest point and place the others evenly to balance out the weight.
Spikes under speaker are a different story. It is much easier to make fine placement adjustments to a speaker with three spikes than with four.