It's important to be aware of the loudness curve, and how the human ear perceives frequencies differently depending on volume. What sounds good quietly won't sound the same when pumped up to dancefloor levels...

It's also a good thing to be aware when one has a frequency loss, due to age or abuse, and factor that into what we do when playing to OTHER people that may not suffer from it. Sure, it means perhaps a compromise in what WE get to hear, but once you are playing for other people, what THEY are hearing should be the main thing, IMO...