equipment on cones

i see many people place amps, cd players and dacs on 3 cones instead of the original feet. they usually set 1 in the mid-front and 2 in the back.
i have experimented with different equipment, and only about half of the time the outcome was better. using other feet/cones than the original is usually better, but not always in the most common 3 cone placement. sometimes it was way better just the other way around, 1 in the back and 2 in the front. But i found the best way that will work most of the time is to use 4 conesjust on the inside of the original feet.
what inside the equipment do one have to think about when placing cones under it? are there any rules? other than the obvious,- place them where it sounds best..