One of the greatest misconceptions about music mixers (especially the great ones) is that they mix at high volume levels. In fact quite the opposite is generally true. Most mixers find that they get better balances that translate well to the real listening world by monitoring at conversation level (79dB SPL) or lower.
High SPL levels for long periods of time are generally not recommended for the following reasons:
1) First the obvious one, exposure to high volume levels for long periods of time my cause long-term physical damage.
2) High volume levels for long periods of time will not only cause the onset of ear fatigue, but general physical fatigue as well. This means that you might effectively only be able to work 6 hours instead of the normal 8 (or 10 or 12) that’s possible with lower levels.
3) The ear has different frequency response curves at high volume levels that overcompensate on both the high and low frequencies. This means that your high volume mix will generally sound pretty limp when played at softer levels.
4) Balances tend to blur at higher levels. What works at higher levels won’t necessarily work when played softer. However, balances that are made at softer levels always work when played louder.
Now this isn’t to say that all mixing should be done at the same level and everything should be played quietly. In fact, music mixers (as opposed to film which always has a constant SPL level) tend to work at a variety of levels; up loud for a minute to check the low end, and moderate while checking the EQ and effects. But the final balances usually will be done quietly.
Don't forget to check out my Music 3.0 blog for tips and tricks on navigating the music business.