There are a few ways to come at this issue. First, many consumer playback systems have really poor sound, in so many different ways. Some have too much treble, some too much low-mid frequencies, and some are band-limited. To make something sound good on every system is nigh impossible, especially because the systems themselves sound bad.
Second, recorded music is exciting, in part, because of the variety. There are infinite possibilities in terms of texture, tone, and dynamics. If we try to optimize the sound of every recording for the lowest common denominator of consumer playback, we run the risk of taking all the risk-taking out of recording. Yuck!
Here are four things to think about to maximize your audio quality while mastering for multiple formats:
Some speakers don't reveal much in the bass, while others are high-end shy. But almost all speakers let you hear the range from 200-4,000 Hz pretty well. This is not to say that the deepest bass or the shimmering highs aren't important. Of course they are. It simply means you should be sure there is enough energy coming through the middle of the mix to be heard well.
Bob Ludwig is a legendary mastering engineer and one of the things he is oh-so-good at is making sure that the important musical information is well heard to most listeners.
Learning to properly control dynamics of the mix elements is an essential skill. Don't be afraid to ride the faders, especially to control the most important elements. A combination of good gain riding and then compression while mixing is the best way to ensure that the mix sounds consistent to the listener on playback. Doing so will help it translate well across systems.
Check the level of the lead instrument in the intro and first verse compared to the last chorus. If the difference is too great, the listener will have trouble with your mix in noisy environments and on small, band-limited speakers.
Too much energy below 80 Hz and too much above 7 kHz will probably cause problems on many systems. Deciding how much is too much is a skill you'll learn over time, but a spectral display can assist you in understanding whether you have boosted too much in the extremes. (See Ozone's EQ module, our Insight metering suite, or other analyzers that show spectral content.) This doesn't mean you should use a high-pass or low-pass filter. That might get rid of a problem, but it will often get rid of some of what is great in the recording, too!
Yes, they can give you a perspective on your sound. But you can't decide what to change, or how much to change, precisely because they sound bad.
To try iZotope's complete mastering solution on your projects, download Ozone.