Generally, if your mixes are sounding completely different on different systems, you have to much information on the fringes. The fundamental sounds of certain things are lying out of range of most systems. for example, the bass and kick's main energy is below 80hz or so and the high end like Hi hats and cymbols and lots of vocal air is above 12khz. This is generally the cause of monitors in the mixing stage. What i've found is that when using monitors that have a hard time reproducing freqs on these fringes, engineers tend to eq from the top or bottom. For example, they set their high end eq lets say on the vocal at around 18khz to give it some air. they start cranking until they hear the effect, but the monitors aren't giving you a real picture up there so by the time you hear this, you've got way to much. when you then play this mix on another system that can reproduce this, well you get the picture. Same goes for the other way. Monitors that have a smiley face curve to them will give you a false sense of air and depth. The way around this is to work inside of these fringes. your mixes will translate better and when it goes to mastering, the ME can deal with content outside of these fringes. Like Thomas said, there is no such thing as a universal translation but in general the mix should just sound like it's coming out of smaller or larger speakers, not change completely.